ASoC: fsl_sai: support more than 2 channels

The FSL SAI can support up to 32 channels using TDM. Report that value so
they can actually be used.

Tested using 8 channels.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>