This patch fixes timer issues in driver disconnect.
It also removes the restriction of one user per channel at a time.

Thanks to Oliver Neukum and Mauro Chehab for finding these issues.
Locking of video stream partly based on saa7134 driver.
Signed-off-by: Dean Anderson <dean@sensoray.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>