ASoC: stm32: sai: fix sysclk management on shutdown

The commit below, adds a call to sysclk callback on shutdown.
This introduces a regression in stm32 SAI driver, as some clock
services are called twice, leading to unbalanced calls.
Move processing related to mclk from shutdown to sysclk callback.
When requested frequency is 0, assume shutdown and release mclk.

Fixes: 2458adb8

 ("SoC: simple-card-utils: set 0Hz to sysclk when shutdown")

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Link: https://lore.kernel.org/r/20191018082040.31022-1-olivier.moysan@st.com


Signed-off-by: Mark Brown <broonie@kernel.org>