greybus: audio: acquire wakelock during active playback

This change was missed while merging original patch
commit-id: 53c765c33f4a69c31027ec012e717d303bd4feca
Thus submitting it again.

Signed-off-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>