drivers/misc/ti-st/st_core.c: fix NULL dereference on protocol type check

drivers/misc/ti-st/st_core.c: fix NULL dereference on protocol type check

If the type we receive is greater than ST_MAX_CHANNELS we can't rely on
type as vector index since we would be accessing unknown memory when we use the type
as index.

 Unable to handle kernel NULL pointer dereference at virtual address 0000001b
 pgd = c0004000
 [0000001b] *pgd=00000000
 Internal error: Oops: 17 [#1] PREEMPT SMP ARM
 Modules linked in: btwilink wl12xx wlcore mac80211 cfg80211 rfcomm bnep bluo
 CPU: 0    Tainted: G        W     (3.4.0+ #15)
 PC is at st_int_recv+0x278/0x344
 LR is at get_parent_ip+0x14/0x30
 pc : [<c03b01a8>]    lr : [<c007273c>]    psr: 200f0193
 sp : dc631ed0  ip : e3e21c24  fp : dc631f04
 r10: 00000000  r9 : 600f0113  r8 : 0000003f
 r7 : e3e21b14  r6 : 00000067  r5 : e2e49c1c  r4 : e3e21a80
 r3 : 00000001  r2 : 00000001  r1 : 00000001  r0 : 600f0113
 Flags: nzCv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
 Control: 10c5387d  Table: 9c50004a  DAC: 00000015

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>