fix build issue on RHEL 7

Author: saurontech

driver/xr_usb_serial_common.c

@@ -1838,7 +1838,12 @@ static int xr_usb_serial_suspend(struct usb_interface *intf, pm_message_t messag
 
 	if (cnt)
 		return 0;
+
+#if LINUX_VERSION_CODE > KERNEL_VERSION(4, 7, 0)
 	if(tty_port_initialized(&xr_usb_serial->port))
+#else
+	if (test_bit(ASYNCB_INITIALIZED, &xr_usb_serial->port.flags))
+#endif
 		stop_data_traffic(xr_usb_serial);
 
 	return 0;
@@ -1858,8 +1863,12 @@ static int xr_usb_serial_resume(struct usb_interface *intf)
 
 	if (cnt)
 		return 0;
-
-	if (tty_port_initialized(&xr_usb_serial->port)) {
+#if LINUX_VERSION_CODE > KERNEL_VERSION(4, 7, 0)
+	if (tty_port_initialized(&xr_usb_serial->port))
+#else
+	if (test_bit(ASYNCB_INITIALIZED, &xr_usb_serial->port.flags))
+#endif
+	{
 		rv = usb_submit_urb(xr_usb_serial->ctrlurb, GFP_NOIO);
 
 		spin_lock_irq(&xr_usb_serial->write_lock);
@@ -1893,7 +1902,12 @@ static int xr_usb_serial_reset_resume(struct usb_interface *intf)
 #else
    	struct tty_struct *tty;
 #endif
-	if (tty_port_initialized(&xr_usb_serial->port)){
+#if LINUX_VERSION_CODE > KERNEL_VERSION(4, 7, 0)
+	if (tty_port_initialized(&xr_usb_serial->port))
+#else
+	if (test_bit(ASYNCB_INITIALIZED, &xr_usb_serial->port.flags))
+#endif
+	{
 #if LINUX_VERSION_CODE > KERNEL_VERSION(3, 9, 0)	
 	tty_port_tty_hangup(&xr_usb_serial->port, false);
 #else