V4L/DVB (12378): uvcvideo: Restructure the driver to support multiple simultaneous streams.

As a first step towards multiple streaming interfaces support, reorganize the
driver's data structures to cleanly separate video control and video streaming
data.

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