If there are still pending requests because no TRB was available,
prepare more when started requests are completed.

Introduce dwc3_gadget_ep_should_continue() to check for incomplete and
pending requests to resume updating new TRBs to the controller's TRB
cache.
Signed-off-by: NThinh Nguyen <thinhn@synopsys.com>
Signed-off-by: NFelipe Balbi <balbi@kernel.org>