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/synclink.c b/drivers/tty/synclink.c
index 666aa14..70e3a52 100644
--- a/drivers/tty/synclink.c
+++ b/drivers/tty/synclink.c
@@ -1359,7 +1359,7 @@
 			}
 		}
 	
-		if ( (info->port.flags & ASYNC_CTS_FLOW) && 
+		if (tty_port_cts_enabled(&info->port) &&
 		     (status & MISCSTATUS_CTS_LATCHED) ) {
 			if (info->port.tty->hw_stopped) {
 				if (status & MISCSTATUS_CTS) {