ALSA: hda - Re-add the lost fake mute support

Yet another regression by the transition to regmap cache; for better
usability, we had the fake mute control using the zero amp value for
Conexant codecs, and this was forgotten in the new hda core code.

Since the bits 4-7 are unused for the amp registers (as we follow the
syntax of AMP_GET verb), the bit 4 is now used to indicate the fake
mute.  For setting this flag, snd_hda_codec_amp_update() becomes a
function from a simple macro.  The bonus is that it gained a proper
function description.

Signed-off-by: Takashi Iwai <tiwai@suse.de>