usb: gadget: pch_udc: Check for DMA mapping error

DMA mapping might fail, we have to check it with dma_mapping_error().
Otherwise DMA-API is not happy:

  DMA-API: pch_udc 0000:02:02.4: device driver failed to check map error[device address=0x00000000027ee678] [size=64 bytes] [mapped as single]

Fixes: abab0c67 ("usb: pch_udc: Fixed issue which does not work with g_serial")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210323153626.54908-3-andriy.shevchenko@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/udc/pch_udc.c

@@ -2946,7 +2946,7 @@ static int init_dma_pools(struct pch_udc_dev *dev)
 	dev->dma_addr = dma_map_single(&dev->pdev->dev, ep0out_buf,
 				       UDC_EP0OUT_BUFF_SIZE * 4,
 				       DMA_FROM_DEVICE);
-	return 0;
+	return dma_mapping_error(&dev->pdev->dev, dev->dma_addr);
 }
 
 static int pch_udc_start(struct usb_gadget *g,