serial: msm_serial: fix comparison of different types

Fix the following sparse warning:

drivers/tty/serial/msm_serial.c:237:37: error: incompatible types in comparison expression (different type sizes)

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>