usb: dwc3: gadget: Remove unnecessary checks

Remove 2 unnecessary checks:
1) A request in the started_list must have its trb field set. So
checking for req->trb is unnecessary.

2) An endpoint must have started (and have not ended) for the request to
still be in the started_list. There's no point to check if the endpoint
is started. We had this check because previously the driver didn't
handle the endpoint's started/ended flags for END_TRANSFER command
properly. See commit 9f45581f ("usb: dwc3: gadget: early giveback
if End Transfer already completed").

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>