usb: dwc3: ep0: Fix endianness of wIndex passed to dwc3_wIndex_to_dep

The wIndex passed in here is CPU endianness, but the function expects
little endian.

Found with sparse.

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