The code enabled SSIEN when triggered by SNDRV_PCM_TRIGGER_START,
so move the disable code to SNDRV_PCM_TRIGGER_STOP for symmetric.

This also allows us to use the SSI driver more flexible so that
it can support some use cases like "aplay S16_LE.wav S24_LE.wav"
which would call the driver in sequence like:
 startup()->hw_params(S16_LE)->trigger(START)->tirgger(STOP)->
 hw_params(S24_LE)->trigger(START)->tirgger(STOP)->shutdown()

If we disable SSIEN in shutdown(), the second hw_params() would
bypass the sample bits setting while using symmetric_rate.
Signed-off-by: Nicolin Chen <b42378@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>