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/net/irda/ircomm/ircomm_tty_attach.c b/net/irda/ircomm/ircomm_tty_attach.c
index 3ab70e7..edab393 100644
--- a/net/irda/ircomm/ircomm_tty_attach.c
+++ b/net/irda/ircomm/ircomm_tty_attach.c
@@ -578,7 +578,7 @@
 	 * will have to wait for the peer device (DCE) to raise the CTS
 	 * line.
 	 */
-	if ((self->port.flags & ASYNC_CTS_FLOW) &&
+	if (tty_port_cts_enabled(&self->port) &&
 			((self->settings.dce & IRCOMM_CTS) == 0)) {
 		IRDA_DEBUG(0, "%s(), waiting for CTS ...\n", __func__ );
 		goto put;