V4L/DVB (5572): Pvrusb2: use mutex instead of semaphore

The pvrusb2 driver use 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: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>