usb: dwc3: gadget: Account for link TRB in TRBs left

The TRBs left calculation didn't account for the link TRB taking up one
spot.

If the trb_dequeue < trb_enqueue, then the result includes the link
TRB slot so it must be adjusted.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>