power: charger-manager: Fix accessing invalidated power supply after charger unbind

The charger manager obtained in probe references to power supplies for
all chargers with power_supply_get_by_name() for later usage. However
if such charger driver was removed then this reference would point to
old power supply (from driver which was removed).

This lead to accessing invalid memory which could be observed with:
$ echo "max77693-charger" > /sys/bus/platform/drivers/max77693-charger/unbind
$ grep . /sys/devices/virtual/power_supply/battery/charger.0/*
$ grep . /sys/devices/virtual/power_supply/battery/*
[   15.339817] Unable to handle kernel paging request at virtual address 0001c12c
[   15.346187] pgd = edd08000
[   15.348814] [0001c12c] *pgd=6dce2831, *pte=00000000, *ppte=00000000
[   15.355075] Internal error: Oops: 80000007 [#1] PREEMPT SMP ARM
[   15.360967] Modules linked in:
[   15.364010] CPU: 2 PID: 1388 Comm: grep Not tainted 3.17.0-next-20141007-00027-ga95e761db1b0 #245
[   15.372859] task: ee03ad00 ti: edcf6000 task.ti: edcf6000
[   15.378241] PC is at 0x1c12c
[   15.381113] LR is at is_ext_pwr_online+0x30/0x6c
[   15.385706] pc : [<0001c12c>]    lr : [<c0339fc4>]    psr: a0000013
[   15.385706] sp : edcf7e88  ip : 00000000  fp : 00000000
[   15.397161] r10: eeb02c08  r9 : c04b1f84  r8 : eeb02c00
[   15.402369] r7 : edc69a10  r6 : eea6ac10  r5 : eea6ac10  r4 : 00000004
[   15.408878] r3 : 0001c12c  r2 : edcf7e8c  r1 : 00000004  r0 : ee914418
[   15.415390] Flags: NzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
[   15.422506] Control: 10c5387d  Table: 6dd0804a  DAC: 00000015
[   15.428236] Process grep (pid: 1388, stack limit = 0xedcf6240)
[   15.434050] Stack: (0xedcf7e88 to 0xedcf8000)
[   15.438395] 7e80:                   ee03ad00 00000000 edcf7f80 eea6aca8 edcf7ec4 c033b7b0
[   15.446554] 7ea0: 00000001 ee1cc3f0 00000004 c06e1e44 eebdc000 c06e1e44 eeb02c00 c0337144
[   15.454713] 7ec0: ee2dac68 c005cffc ee1cc3c0 c06e1e44 00000fff 00001000 eebdc000 c0278ca8
[   15.462872] 7ee0: c0278c8c ee1cc3c0 eeb7ce00 c014422c edcf7f20 00008000 ee1cc3c0 ee9a48c0
[   15.471030] 7f00: 00000001 00000001 edcf7f80 c0142d94 c0142d70 c01060f4 00021000 ee1cc3f0
[   15.479190] 7f20: 00000000 00000000 c06a2150 eebdc000 2e7ec000 ee9a48c0 00008000 00021000
[   15.487349] 7f40: edcf7f80 00008000 edcf6000 00021000 00021000 c00e39a4 00000000 ee9a48c0
[   15.495508] 7f60: 00004000 00000000 00000000 ee9a48c0 ee9a48c0 00008000 00021000 c00e3aa0
[   15.503668] 7f80: 00000000 00000000 0001f2e0 0001f2e0 00021000 00001000 00000003 c000f364
[   15.511826] 7fa0: 00000000 c000f1a0 0001f2e0 00021000 00000003 00021000 00008000 00000000
[   15.519986] 7fc0: 0001f2e0 00021000 00001000 00000003 00000001 000205e8 00000000 00021000
[   15.528145] 7fe0: 00008000 bebbe910 0000a7ad b6edc49c 60000010 00000003 aaaaaaaa aaaaaaaa
[   15.536320] [<c0339fc4>] (is_ext_pwr_online) from [<c033b7b0>] (charger_get_property+0x170/0x314)
[   15.545164] [<c033b7b0>] (charger_get_property) from [<c0337144>] (power_supply_show_property+0x48/0x20c)
[   15.554719] [<c0337144>] (power_supply_show_property) from [<c0278ca8>] (dev_attr_show+0x1c/0x48)
[   15.563577] [<c0278ca8>] (dev_attr_show) from [<c014422c>] (sysfs_kf_seq_show+0x84/0x104)
[   15.571725] [<c014422c>] (sysfs_kf_seq_show) from [<c0142d94>] (kernfs_seq_show+0x24/0x28)
[   15.579973] [<c0142d94>] (kernfs_seq_show) from [<c01060f4>] (seq_read+0x1b0/0x484)
[   15.587614] [<c01060f4>] (seq_read) from [<c00e39a4>] (vfs_read+0x88/0x144)
[   15.594552] [<c00e39a4>] (vfs_read) from [<c00e3aa0>] (SyS_read+0x40/0x8c)
[   15.601417] [<c00e3aa0>] (SyS_read) from [<c000f1a0>] (ret_fast_syscall+0x0/0x48)
[   15.608877] Code: bad PC value
[   15.611991] ---[ end trace a88fcc95208db283 ]---

The charger-manager should get reference to charger power supply on
each use of get_property callback.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: <stable@vger.kernel.org>
Fixes: 3bb3dbbd56ea ("power_supply: Add initial Charger-Manager driver")
Signed-off-by: Sebastian Reichel <sre@kernel.org>