V4L/DVB (5766): ET61x251 driver updates

- Make the driver depend on V4L2 only (KConfig)
- Better and safe locking mechanism of the device structure on open(),
  close() and disconnect()
- Use kref for handling device deallocation
- Generic cleanups

Signed-off-by: Luca Risolia <luca.risolia@studio.unibo.it>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>