ASoC: fsl_ssi: Implement symmetric_channels and symmetric_samplebits

Since we introduced symmetric_channels and symmetric_samplebits, we implement
these two features to fsl_ssi so as to drop some no-more-needed code and make
the driver neat and clean.

Signed-off-by: Nicolin Chen <b42378@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>