V4L/DVB (4613): Unmute/mute saa7134 when opening/closing the audio capture device.

This patch should enable unmuting the audio device when opening it (and
posterior muting when closing it), doing away with the need for unmute ioctls
or v4lctl usage.

Signed-off-by: Ricardo Cerqueira <v4l@cerqueira.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>