ASoC: fsl_audmix: make clock and output src write only

"alsactl -f state.conf store/restore" sequence fails because setting
"mixing clock source" and "output source" requires active TDM clock
being started for configuration propagation. Make these two controls
write only so that their values are not stored at "alsactl store".

Signed-off-by: Viorel Suman <viorel.suman@nxp.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Link: https://lore.kernel.org/r/1600104274-13110-1-git-send-email-viorel.suman@oss.nxp.com


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