usb: gadget: mv_u3d: mv_u3d_start_queue() refactoring

The patch improves readability of mv_u3d_start_queue()
by rearranging its code with two semantic modifications:
- assignment zero to ep->processing if usb_gadget_map_request() fails;
- propagation of error code from mv_u3d_req_to_trb() instead of
  hardcoded -ENOMEM.

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