acp: reset soft_volume

Reset the software volume when initializing the device and when the
hardware volume is updated.

The software volume was set to 0 by default and if the session manager
did not change the volume enough to cause a software volume change,
it would remain 0 and there would be silence.

Also improve the debug a little.

See #1160 #1167 #1164 #1049 #1117