By returning the number of TRBs prepared, we know whether to execute
__dwc3_gadget_kick_transfer(). This allows us to check if we ran out of
TRBs when extra TRBs are needed for OUT transfers. It also allows us to
properly handle usb_gadget_map_request_by_dev() error.

Fixes: c6267a51 ("usb: dwc3: gadget: align transfers to wMaxPacketSize")
Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>