media: uvcvideo: Rename uvc_format 'frame' field to 'frames'

The uvc_format 'frame' field points to an array of frames. Rename it to
'frames' to make this clearer.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>