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>
diff --git a/drivers/tty/serial/msm_serial.c b/drivers/tty/serial/msm_serial.c
index 252d514..8f58540 100644
--- a/drivers/tty/serial/msm_serial.c
+++ b/drivers/tty/serial/msm_serial.c
@@ -234,7 +234,8 @@
 			break;
 
 		if (msm_port->is_uartdm)
-			num_chars = min(tx_count - tf_pointer, sizeof(buf));
+			num_chars = min(tx_count - tf_pointer,
+					(unsigned int)sizeof(buf));
 		else
 			num_chars = 1;