ASoC: wm_adsp: Avoid narrow race condition on compr pointer

There is a very small window between then wm_adsp_compr_free gets call
and when the DSP is actually powered down. If we get an IRQ from the DSP
in this window then the wm_adsp_compr pointer will be NULL. This patch
adds a check for this into the IRQ handler to avoid any issues when this
happens.

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