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

drivers/tty/serial/msm_serial.c

@@ -234,7 +234,8 @@ static void handle_tx(struct uart_port *port)
 			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;