ASoC: wm_adsp: Check return value from wm_adsp_buffer_init

We are not checking the return value from a call to wm_adsp_buffer_init
it looks like this used to be returned at the bottom of the function but
probably got missed when more error paths were added. This patch adds
the appropriate error check.

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