ASoC: dapm: Consolidate MUXs and value MUXs

MUXs and value MUXs are almost identical, the only difference is that a value
MUX uses a look-up table to map from the selected control item to a register
value, while MUXs use a direct mapping. This patch uses
snd_soc_enum_item_to_val() and snd_soc_enum_val_to_item(), which where earlier
introduced during the consolidation of enum and value enum controls, to hide
this difference. This allows us to use the same code path for both MUXs and
value MUXs and a lot of nearly duplicated code can be removed.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>