ASoC: mxs-saif: Store saif state

Trigger commands may be passed multiple times. To avoid errors with
clk_enable/disable, store the saif state and return if saif is already
running/stopped.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>