[ALSA] Revolution 5.1 - complete the AK5365 support

Complete the AK5365 support.
This adds a boolean control to toggle the soft mute feature of the
AK5365 chip.

Signed-off-by: Jochen Voss <voss@seehuhn.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>