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/isicom.c b/drivers/tty/isicom.c
index 99cf22e..d7492e1 100644
--- a/drivers/tty/isicom.c
+++ b/drivers/tty/isicom.c
@@ -600,7 +600,7 @@
 					port->status &= ~ISI_DCD;
 			}
 
-			if (port->port.flags & ASYNC_CTS_FLOW) {
+			if (tty_port_cts_enabled(&port->port)) {
 				if (tty->hw_stopped) {
 					if (header & ISI_CTS) {
 						port->port.tty->hw_stopped = 0;