ASoC: SDCA: support Q7.8 volume format

The SDCA specification uses Q7.8 volume format.
This patch adds a field to indicate whether it is SDCA volume control
and supports the volume settings.

Signed-off-by: Shuming Fan <shumingf@realtek.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://patch.msgid.link/20251106093335.1363237-1-shumingf@realtek.com
Signed-off-by: Mark Brown <broonie@kernel.org>