ASoC: simple-mux: add read function

During initialisation DAPM tries to read the state of the MUX
being connected, resulting in this error log:
input-mux: ASoC: error at soc_component_read_no_lock on input-mux: -5

Provide a read function which allows DAPM to read the state of the
MUX.

Signed-off-by: Maarten Zanders <maarten.zanders@mind.be>
Link: https://lore.kernel.org/r/20221028191303.166115-1-maarten.zanders@mind.be


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