ALSA: scarlett2: Add R/O headphone volume control

The Scarlett 4i4 Gen 4 adds a R/O headphone volume control in addition
to a R/O master volume control (which is already supported).

Mark the new scarlett2_notify_volume() function with __always_unused
until it gets used when the Gen 4 notification callback function arrays
are added.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/bd4a76da157f8cc3fbfa02eba96d02bdb86817c5.1703612638.git.g@b4.vu