V4L/DVB (5809): Use mutex instead of semaphore in Philips webcam driver

The Philips webcam driver uses a semaphore as mutex. Use the mutex API
instead of the (binary) semaphore.
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Signed-off-by: Matthias Kaehlcke <matthias.kaehlcke@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>