usb: dwc3: Reset num_trbs after skipping

Currently req->num_trbs is not reset after the TRBs are skipped and
processed from the cancelled list. The gadget driver may reuse the
request with an invalid req->num_trbs, and DWC3 will incorrectly skip
trbs. To fix this, simply reset req->num_trbs to 0 after skipping
through all of them.

Fixes: c3acd590 ("usb: dwc3: gadget: use num_trbs when skipping TRBs on ->dequeue()")
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>