ASoC: Update WM9081 for tdm_slot() API change

Store the TDM slot width then if it's set use that rather than the
sample size to calculate BCLK. Leave imposing constraints to the
core (which should do this but doesn't yet) or machine driver.

Also allow 0 TDM slots to be configure (for use when disabling TDM).

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>