usb: dwc3: gadget: always kick if num_pending_sgs > 0

When we get a half-way processed request, we should
make sure to try to prepare further TRBs for it or
for any possibly queued up request held in our
pending_list. This will make sure our controller is
kept busy for as long as possible.

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