ALSA: asihpi - Change compander API and tidy

Compander API changed to one function per parameter.
Factor out some common code for stereo log value reading.
Make some more entity functions static.

Signed-off-by: Eliot Blennerhassett <eblennerhassett@audioscience.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>