V4L/DVB (5997): cx25840: fix audio mute handling and reporting

Audio muting for the tuner input was implemented by stopping the 
audio microcontroller and restarting it on unmute. However, it 
appears that this method can actually crash the audio firmware. 
It's rare and seems to happen with NTSC only.

It has been reimplemented by setting to volume to 0. In addition, the
reporting of the mute state has been improved as well: it used to be
impossible to detect whether the audio was muted by the user or if it
was muted due to the microcontroller trying to detect the audio
standard. This is now clearly stated.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>