usb: dwc3: gadget: add sg and num_pending_sgs to dwc3_request

These two fields will be used in a follow-up patch
to track how many entries of request's sglist we
have already processed. The reason is that if a
gadget driver sends an sglist with more entries then
we can fit in the ring, we will have to continue
processing remaining afterwards.

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