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/serial/mxs-auart.c b/drivers/tty/serial/mxs-auart.c
index 3a667ee..dafeef2 100644
--- a/drivers/tty/serial/mxs-auart.c
+++ b/drivers/tty/serial/mxs-auart.c
@@ -262,7 +262,7 @@
 
 	ctrl &= ~AUART_CTRL2_RTSEN;
 	if (mctrl & TIOCM_RTS) {
-		if (u->state->port.flags & ASYNC_CTS_FLOW)
+		if (tty_port_cts_enabled(&u->state->port))
 			ctrl |= AUART_CTRL2_RTSEN;
 	}