ASoC: wm_adsp: Allow preloader to do the final shutdown of the DSP

The booting process for the DSP is clearly separated into two parts, the
preloader brings up the core and downloads code, then the main widget
starts the code actually executing. However the shutdown sequence is all
handled with the main widget.

To allow the preloading to be run independently of the main audio bring
up it makes sense, and is generally just cleaner, for the preloader
widget to shutdown those things it initialised. This patch moves the
appropriate parts of the shutdown process into the preloader widget.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>