mtd: jedec_probe: Fix crash in jedec_read_mfr()

mtd: jedec_probe: Fix crash in jedec_read_mfr()

commit 87a73eb5b56fd6e07c8e499fe8608ef2d8912b82 upstream.

It turns out that the loop where we read manufacturer
jedec_read_mfd() can under some circumstances get a
CFI_MFR_CONTINUATION repeatedly, making the loop go
over all banks and eventually hit the end of the
map and crash because of an access violation:

Unable to handle kernel paging request at virtual address c4980000
pgd = (ptrval)
[c4980000] *pgd=03808811, *pte=00000000, *ppte=00000000
Internal error: Oops: 7 [#1] PREEMPT ARM
CPU: 0 PID: 1 Comm: swapper Not tainted 4.16.0-rc1+ #150
Hardware name: Gemini (Device Tree)
PC is at jedec_probe_chip+0x6ec/0xcd0
LR is at 0x4
pc : [<c03a2bf4>]    lr : [<00000004>]    psr: 60000013
sp : c382dd18  ip : 0000ffff  fp : 00000000
r10: c0626388  r9 : 00020000  r8 : c0626340
r7 : 00000000  r6 : 00000001  r5 : c3a71afc  r4 : c382dd70
r3 : 00000001  r2 : c4900000  r1 : 00000002  r0 : 00080000
Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
Control: 0000397f  Table: 00004000  DAC: 00000053
Process swapper (pid: 1, stack limit = 0x(ptrval))

Fix this by breaking the loop with a return 0 if
the offset exceeds the map size.

Fixes: 5c9c11e1c47c ("[MTD] [NOR] Add support for flash chips with ID in bank other than 0")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>