net:stmmac: fix broken stmmac_pltfr_remove.

This patch fixes stmmac_pltfr_remove function, which is broken because,
it is accessing plat variable via freed memory priv pointer which gets
freed by free_netdev called from stmmac_dvr_remove.

In short this patch caches the plat pointer in local variable before
calling stmmac_dvr_remove to prevent code accessing freed memory.

Without this patch any attempt to remove the stmmac device will fail as
below:

Unregistering eth 0 ...
Unable to handle kernel paging request at virtual address 6b6b6bab
pgd = de5dc000
[6b6b6bab] *pgd=00000000
Internal error: Oops: 5 [#1] PREEMPT SMP
Modules linked in: cdev(O+)
CPU: 0    Tainted: G           O  (3.3.1_stm24_0210-b2000+ #25)
PC is at stmmac_pltfr_remove+0x2c/0xa0
LR is at stmmac_pltfr_remove+0x28/0xa0
pc : [<c01b8908>]    lr : [<c01b8904>]    psr: 60000013
sp : def6be78  ip : de6c5a00  fp : 00000000
r10: 00000028  r9 : c082d81d  r8 : 00000001
r7 : de65a600  r6 : df81b240  r5 : c0413fd8  r4 : 00000000
r3 : 6b6b6b6b  r2 : def6be6c  r1 : c0355e2b  r0 : 00000020
Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
Control: 10c53c7d  Table: 5e5dc04a  DAC: 00000015
Process insmod (pid: 738, stack limit = 0xdef6a2f0)
Stack: (0xdef6be78 to 0xdef6c000)
be60:                                                       c0413fe0
c0403658
be80: c0400bb0 c019270c c01926f8 c0191478 00000000 c0414014 c0413fe0
c01914d8
bea0: 00000000 c0413fe0 df8045d0 c019109c c0413fe0 c0400bf0 c0413fd8
c018f04c
bec0: 00000000 bf000000 c0413fd8 c01929a0 c0413fd8 bf000000 00000000
c0192bfc
bee0: bf00009c bf000014 def6a000 c000859c 00000000 00000001 bf00009c
bf00009c
bf00: 00000001 bf00009c 00000001 bf0000e4 de65a600 00000001 c082d81d
c0058cd0
bf20: bf0000a8 c004fbd8 c0056414 c082d815 c02aea20 bf0001f0 00b0b008
e0846208
bf40: c03ec8a0 e0846000 0000db0d e0850604 e08504de e0853a24 00000204
000002d4
bf60: 00000000 00000000 0000001c 0000001d 00000009 00000000 00000006
00000000
bf80: 00000003 f63d4e2e 0000db0d bef02ed8 00000080 c000d2e8 def6a000
00000000
bfa0: 00000000 c000d140 f63d4e2e 0000db0d 00b0b018 0000db0d 00b0b008
b6f4f298
bfc0: f63d4e2e 0000db0d bef02ed8 00000080 00000003 00000000 00010000
00000000
bfe0: 00b0b008 bef02c64 00008d20 b6ef3784 60000010 00b0b018 5a5a5a5a
5a5a5a5a
[<c01b8908>] (stmmac_pltfr_remove+0x2c/0xa0) from [<c019270c>]
(platform_drv_remove+0x14/0x18)
[<c019270c>] (platform_drv_remove+0x14/0x18) from [<c0191478>]
(__device_release_driver+0x64/0xa4)
[<c0191478>] (__device_release_driver+0x64/0xa4) from [<c01914d8>]
(device_release_driver+0x20/0x2c)
[<c01914d8>] (device_release_driver+0x20/0x2c) from [<c019109c>]
(bus_remove_device+0xcc/0xdc)
[<c019109c>] (bus_remove_device+0xcc/0xdc) from [<c018f04c>]
(device_del+0x104/0x160)
[<c018f04c>] (device_del+0x104/0x160) from [<c01929a0>]
(platform_device_del+0x18/0x58)
[<c01929a0>] (platform_device_del+0x18/0x58) from [<c0192bfc>]
(platform_device_unregister+0xc/0x18)
[<c0192bfc>] (platform_device_unregister+0xc/0x18) from [<bf000014>]
(r_init+0x14/0x2c [cdev])
[<bf000014>] (r_init+0x14/0x2c [cdev]) from [<c000859c>]
(do_one_initcall+0x90/0x160)
[<c000859c>] (do_one_initcall+0x90/0x160) from [<c0058cd0>]
(sys_init_module+0x15c4/0x1794)
[<c0058cd0>] (sys_init_module+0x15c4/0x1794) from [<c000d140>]
(ret_fast_syscall+0x0/0x30)
Code: e1a04000 e59f0070 eb039b65 e59636e4 (e5933040)

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>