ASoC: wm8994: Support constraining the maximum number of channels clocked

Some systems use the audio CODEC to clock a DAI with multiple data lines
in parallel, meaning that bit clocks are only required for a smaller number
of channels than data is sent for. In some cases providing the extra bit
clocks can take the other devices on the audio bus out of spec.

Support such systems by allowing a maximum number of channels to be
specified.

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