ASoC: fsl_ssi: Set default slot number for common cases

For those platforms using DAI master mode like I2S, it's better to pre-set
a default slot number so that there's no need for these common cases to set
the slot number from its machine driver any more.

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