[media] uvcvideo: Move fields from uvc_buffer::buf to uvc_buffer

Add mem, length and bytesused fields to the uvc_buffer structure and use
them instead of accessing the uvc_buffer::buf m.offset, length and
bytesused fields directly. This prepares the driver to the conversion to
videobuf2.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>