usb: gadget: mv_u3d: add check for dma mapping error

mv_u3d_req_to_trb() does not check for dma mapping errors.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>