usb: dwc3: gadget: Keep TRBs in request order

If we couldn't finish preparing all the TRBs of a request, don't prepare
the next request. Otherwise, the TRBs order will be mixed up and the
controller will process the wrong TRB. This is a corner case where
there's not enough TRBs for a request that needs the extra TRB but
there's still 1 available TRB in the pool.

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