ASoC: fsl ssi doc: Remove unused properties

The fsl_ssi driver only checks for the ac97 mode property, so remove the unused
ones.

Suggested-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Timur Tabi <timur@tabi.org>
Signed-off-by: Mark Brown <broonie@kernel.org>