ASoC: fsl_sai: Use snd_pcm_format_width()

Use common helper function snd_pcm_format_width() to make code neater.

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>