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.

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>
Signed-off-by: Gigi Joseph <gigi.joseph@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>