usb: video: Fix endianness mismatches in descriptor structures

All UVC descriptors use little-endian format, update the data structures
accordingly.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Tested-by: Paul Elder <paul.elder@ideasonboard.com>