usb: dwc3: gadget: initialize transfer index from send_gadget_ep_cmd()

Instead of *always* calling dwc3_gadget_ep_get_transfer_index() after
sending a Start Transfer command, we can call it once from
dwc3_send_gadget_ep_cmd() itself.

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