usb: dwc3: gadget: Correct the logic for queuing sgs

The present code correctly fetches the req which were previously not
queued from the started_list but fails to continue queuing from the sg
where it previously stopped queuing (because of the unavailable TRB's).
This patch correct's the code to continue queuing from the correct sg
present in the sglist.

For example, consider 5 sgs in req. Because of limited TRB's among the
5 sgs only 3 got queued. This patch corrects the code to start queuing
from correct sg i.e 4th sg when the TRBs are available.

Signed-off-by: Anurag Kumar Vulisha <anuragku@xilinx.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>