ASoC: fsl_esai: Use dynamic slot width as default

The driver previously used 32-bit fixed slot width as default.
In result, ESAI might use 32-bit length to capture 16-bit width
audio slot from CODEC side when ESAI is running as DAI slave.

So this patch just removes the default slot_width so as to use
dynamic slot width. If there comes a specific situation that
needs a fixed width, the machine driver shall set slot_width
via set_tdm_slot() so as to let the ESAI driver replace the
dynamic width policy with the fixed value.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>