staging: bcm2835-audio: Fix mute controls, volume handling cleanup

staging: bcm2835-audio: Fix mute controls, volume handling cleanup

In the current code, the mute control is dealt in a special manner,
modifying the current volume and saving the old volume, etc.  This is
inconsistent (e.g. change the volume while muted, then unmute), and
way too complex.

Also, the whole volume handling code has conversion between ALSA
volume and raw volume values, which can lead to another
inconsistency and complexity.

This patch simplifies these points:
- The ALSA volume value is saved in chip->volume
- volume->mute saves the mute state
- The mute state is evaluated only when the actual volume is passed to
  the hardware, bcm2835_audio_set_ctls()

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>