USB: quatech2: only write to the tty if the port is open.

The commit 2e124b4a390ca85325fae75764bef92f0547fa25 removed the checks
that prevented qt2_process_read_urb() from trying to put chars into
ttys that weren't actually opened.  This resulted in 'tty is NULL'
warnings from flush_to_ldisc() when the device was used.

The devices use just one read urb for all ports.  As a result
qt2_process_read_urb() may be called with the current port set to a
port number that has not been opened.  Add a check if the port is open
before calling tty_flip_buffer_push().

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/quatech2.c

@@ -661,7 +661,9 @@ void qt2_process_read_urb(struct urb *urb)
 						 __func__);
 					break;
 				}
-				tty_flip_buffer_push(&port->port);
+
+				if (port_priv->is_open)
+					tty_flip_buffer_push(&port->port);
 
 				newport = *(ch + 3);
 
@@ -704,7 +706,8 @@ void qt2_process_read_urb(struct urb *urb)
 		tty_insert_flip_string(&port->port, ch, 1);
 	}
 
-	tty_flip_buffer_push(&port->port);
+	if (port_priv->is_open)
+		tty_flip_buffer_push(&port->port);
 }
 
 static void qt2_write_bulk_callback(struct urb *urb)