usb: dwc3: gadget: reset resource index to zero

If we collected two requests together (i.e. only the last of them has
LST=1) then we only have to stop transfer once: The clean-up code will
cleanup everything until first TRB with the LST bit set.
After XferComplete this index should be no longer valid since there is
no transfer pending.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>