serial: add a new helper function

In most of the time, the driver needs to check if the cts flow control
is enabled. But now, the driver checks the ASYNC_CTS_FLOW flag manually,
which is not a grace way. So add a new wraper function to make the code
tidy and clean.

Signed-off-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/tty/mxser.c b/drivers/tty/mxser.c
index bb2da4c..cfda47d 100644
--- a/drivers/tty/mxser.c
+++ b/drivers/tty/mxser.c
@@ -830,7 +830,7 @@
 			wake_up_interruptible(&port->port.open_wait);
 	}
 
-	if (port->port.flags & ASYNC_CTS_FLOW) {
+	if (tty_port_cts_enabled(&port->port)) {
 		if (tty->hw_stopped) {
 			if (status & UART_MSR_CTS) {
 				tty->hw_stopped = 0;