drivers:misc:ti-st: protect against bad packets

We encounter situations where we got bad packet type from the
UART (probably due to platform problem or UART driver issues)
which caused us out of boundary array access,
which eventually led to kernel panic.

Ported from:
	p-android-omap-3.0:89b502674ca [Amir Ayun <amira@ti.com>]

Change-Id: I6f0f13168c69e46260c5abb7d7c0b854c61cab7d
Signed-off-by: Amir Ayun <amira@ti.com>
Signed-off-by: Pavan Savoy <pavan_savoy@ti.com>
Signed-off-by: Leonid Iziumtsev <x0153368@ti.com>