ASoC: codecs: rt5670: use SOC_VALUE_ENUM_SINGLE_DECL for DAC2 L/R MX-1B

ASoC: codecs: rt5670: use SOC_VALUE_ENUM_SINGLE_DECL for DAC2 L/R MX-1B

DAC2 L/R source selection fields (MX-1B [6:4] and [2:0]) contain non
contiguous values due to reserved bits documented in datasheet (page 66):
<https://www.elinfor.com/pdf/RealtekMicroelectronics/ALC5670-VB-
RealtekMicroelectronics.pdf>

Switch from SOC_ENUM_SINGLE_DECL to SOC_VALUE_ENUM_SINGLE_DECL
to handle discrete values.

Signed-off-by: Alex Tran <alex.t.tran@gmail.com>
Link: https://patch.msgid.link/20250929004625.1310721-1-alex.t.tran@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>