tty: mxs-auart: fix a potential NULL pointer dereference

In case ioremap fails, the fix returns -ENOMEM to avoid NULL
pointer dereferences.
Multiple places use port.membase.
Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/mxs-auart.c

@@ -1686,6 +1686,10 @@ static int mxs_auart_probe(struct platform_device *pdev)
 
 	s->port.mapbase = r->start;
 	s->port.membase = ioremap(r->start, resource_size(r));
+	if (!s->port.membase) {
+		ret = -ENOMEM;
+		goto out_disable_clks;
+	}
 	s->port.ops = &mxs_auart_ops;
 	s->port.iotype = UPIO_MEM;
 	s->port.fifosize = MXS_AUART_FIFO_SIZE;