ASoC: soc-pcm: msbits constraint: Drop 8 and 16 bit sample sizes

As per discussion we can safely ignore the 8 and 16 bit sample
sizes when applying the msbits constraint.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>