ASoC: rt715-sdca: volume step modification

Volume step (dB/step) modification to fix format error
which shown in amixer control.

Signed-off-by: Jack Yu <jack.yu@realtek.com>
Link: https://lore.kernel.org/r/b1f546ad16dc4c7abb7daa7396e8345c@realtek.com


Signed-off-by: Mark Brown <broonie@kernel.org>