This allows changing the volume of each digital input/output
independently, and provides the only "master volume" for the DAC.
(The ADC also has a gain control on the analog side.)

While the hardware supports digital gain up to +72dB, the controls here
are limited to +24dB maximum, as any gain above that level makes volume
sliders difficult to use, and is extremely likely to cause clipping.
Signed-off-by: NSamuel Holland <samuel@sholland.org>
Link: https://lore.kernel.org/r/20211118033645.43524-1-samuel@sholland.orgSigned-off-by: NMark Brown <broonie@kernel.org>