usb: dwc3: gadget: init req->{direction,epnum} from alloc_request()

We dont' need to touch req->direction or req->epnum from
ep_queue(). It's enough that we initialize both fields from
alloc_request() and just keep them for the entire lifetime of the
request.

No functional changes.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>