ASoC: wm_adsp: Remove redundant NULL check in wm_adsp_buffer_free

wm_adsp_compr_detach is NULL aware so there is no need to check for NULL
before calling it, remove the redundant check.

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