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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>