V4L/DVB (5811): Use mutex instead of semaphore in Virtual Video driver

The Virtual Video driver uses a semaphore as mutex. Use the mutex API
instead of the (binary) semaphore.

Signed-off-by: Matthias Kaehlcke <matthias.kaehlcke@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>