ALSA: oxygen: add self-documenting functions

Introduce some trivial functions to better document the relationships of
the various model callbacks.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>

diff --cc sound/pci/oxygen/hifier.c
Simple merge

diff --cc sound/pci/oxygen/oxygen.c
Simple merge

diff --cc sound/pci/oxygen/virtuoso.c
Simple merge