usb: dwc3: gadget: tracking per-TRB remaining bytes

This will give us a simpler way of figuring out how
many bytes were left in each TRB. It's useful for
cases where we queue only part of an SG-list due to
amount of available TRBs at the time of kicking the
transfer.

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