usb: dwc3: gadget: issue ENDTRANSFER conditional on resource_index

Because of recent changes to transfer handling on
DWC3, we will not get XferComplete unless we
completely fill up our TRB ring. This means that we
might get a Reset or Disconnect without getting a
XferComplete first.

In order to correctly release our allocated Transfer
Resource, we must issue ENDTRANSFER command whenever
dep->resource_index is valid.

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