ASoC: sigmadsp: Disable cache mechanism for readbacks

The ALSA control readback functionality only works for non-volatile
controls, i.e. control values that does not change on their own without
driver interaction.

This doesn't work for readbacks since the DSP firmware updates the
control value. Disable the cache mechanism in the driver if the control
name matches the prefix used for readbacks to ensure that the control
value is valid.

Signed-off-by: Niklas Carlsson <niklasc@axis.com>
Link: https://lore.kernel.org/r/20210419144901.9441-1-Niklas.Carlsson@axis.com
Signed-off-by: Mark Brown <broonie@kernel.org>