ASoC: fsl_ssi: Remove FSLSSI_I2S_RATES definition

The comment for the FSLSSI_I2S_RATES definition states that the
driver currently only supports I2S slave mode, which is no longer
correct.

As FSLSSI_I2S_RATES is the same as the standard SNDRV_PCM_RATE_CONTINUOUS,
just remove its definition and its comments to make the code simpler.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Timur Tabi <timur@tabi.org>
Signed-off-by: Mark Brown <broonie@kernel.org>