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/amiserial.c b/drivers/tty/amiserial.c
index 2b7535d..42d0a25 100644
--- a/drivers/tty/amiserial.c
+++ b/drivers/tty/amiserial.c
@@ -420,7 +420,7 @@
 				tty_hangup(port->tty);
 		}
 	}
-	if (port->flags & ASYNC_CTS_FLOW) {
+	if (tty_port_cts_enabled(port)) {
 		if (port->tty->hw_stopped) {
 			if (!(status & SER_CTS)) {
 #if (defined(SERIAL_DEBUG_INTR) || defined(SERIAL_DEBUG_FLOW))