s390/3270: fix return value check in tty3270_resize_work()

In case of error, the function tty3270_alloc_screen() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/tty3270.c

@@ -810,7 +810,7 @@ static void tty3270_resize_work(struct work_struct *work)
 	struct winsize ws;
 
 	screen = tty3270_alloc_screen(tp->n_rows, tp->n_cols);
-	if (!screen)
+	if (IS_ERR(screen))
 		return;
 	/* Switch to new output size */
 	spin_lock_bh(&tp->view.lock);