usb: dwc3: gadget: Wrap around when skip TRBs

When skipping TRBs, we need to account for wrapping around the ring
buffer and not modifying some invalid TRBs. Without this fix, dwc3 won't
be able to check for available TRBs.

Cc: stable <stable@vger.kernel.org>
Fixes: 7746a8df

 ("usb: dwc3: gadget: extract dwc3_gadget_ep_skip_trbs()")
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>