ASoC: fsl_sai: Drop useless channels check in hw_params()

SAi only supports two data channels on hardware level and the driver also does
register the min->1 and max->2, so no need to check channels.

Signed-off-by: Nicolin Chen <Guangyu.Chen@freescale.com>
Reviewed-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>