usb: dwc3: gadget: assign resource_index inside get_transfer_index()

Instead of returning resource index number just to assign it to a
field inside 'dep' which was passed as argument, we can assing
dep->resource_index from inside dwc3_gadget_ep_get_transfer_index()
itself.

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