ASoC: fsl-ssi: Fix register values when disabling

The bits we have to clear when disabling are different when the other
stream is still active.

This patch fixes the calculation of new register values after disabling
one stream. It also adds a more detailed description of the new register
value calculation.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Tested-By: Michael Grzeschik <mgr@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>