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/cyclades.c b/drivers/tty/cyclades.c
index e3954da..0a6a0bc 100644
--- a/drivers/tty/cyclades.c
+++ b/drivers/tty/cyclades.c
@@ -727,7 +727,7 @@
 		else
 			tty_hangup(tty);
 	}
-	if ((mdm_change & CyCTS) && (info->port.flags & ASYNC_CTS_FLOW)) {
+	if ((mdm_change & CyCTS) && tty_port_cts_enabled(&info->port)) {
 		if (tty->hw_stopped) {
 			if (mdm_status & CyCTS) {
 				/* cy_start isn't used