media: uvcvideo: Rename uvc_streaming 'format' field to 'formats'

The uvc_streaming 'format' field points to an array of formats. Rename
it to 'formats' 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>