ASoC: wm_adsp: Support streams which can start/stop with DSP active

Clear the buffer data structure on each trigger start such that the
buffer is in a sensible state even if the DSP itself didn't restart.
This is necessary to support voice control streams which can trigger
multiple times without reloading the firmware.

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