audio: split ctl_* functions into enable_* and volume_*

This way we no longer need vararg functions, improving compile time
error detection.  Also now it's possible to check actually what commands
are supported, without needing to manually update ctl_caps.

Signed-off-by: Kővágó, Zoltán <DirtY.iCE.hu@gmail.com>
Message-id: 2b08b3773569c5be055d0a0fb2f29ff64e79f0f4.1568927990.git.DirtY.iCE.hu@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>