8250_lpss: check null return when calling pci_ioremap_bar

pci_ioremap_bar may return null. This is eventually de-referenced at
drivers/dma/dw/core.c:1154 and drivers/dma/dw/core.c:1168. A null check
is needed to prevent null de-reference. I am adding the check and in
case of failure. Thanks to Andy Shevchenko for the hint on the necessity
of pci_iounmap when exiting.
Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20190719174848.24216-1-navid.emamdoost@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_lpss.c

@@ -169,10 +169,12 @@ static void qrk_serial_setup_dma(struct lpss8250 *lpss, struct uart_port *port)
 	struct pci_dev *pdev = to_pci_dev(port->dev);
 	int ret;
 
+	chip->pdata = &qrk_serial_dma_pdata;
 	chip->dev = &pdev->dev;
 	chip->irq = pci_irq_vector(pdev, 0);
 	chip->regs = pci_ioremap_bar(pdev, 1);
-	chip->pdata = &qrk_serial_dma_pdata;
+	if (!chip->regs)
+		return;
 
 	/* Falling back to PIO mode if DMA probing fails */
 	ret = dw_dma_probe(chip);
@@ -195,11 +197,15 @@ static void qrk_serial_setup_dma(struct lpss8250 *lpss, struct uart_port *port)
 
 static void qrk_serial_exit_dma(struct lpss8250 *lpss)
 {
+	struct dw_dma_chip *chip = &lpss->dma_chip;
 	struct dw_dma_slave *param = &lpss->dma_param;
 
 	if (!param->dma_dev)
 		return;
-	dw_dma_remove(&lpss->dma_chip);
+
+	dw_dma_remove(chip);
+
+	pci_iounmap(to_pci_dev(chip->dev), chip->regs);
 }
 #else	/* CONFIG_SERIAL_8250_DMA */
 static void qrk_serial_setup_dma(struct lpss8250 *lpss, struct uart_port *port) {}