greybus: audio: acquire wakelock during active playback

use pm_stay_awake & pm_relax to avoid suspend sequence during
active playback

testing Done:
Music Playback ongoing
$ cat /sys/devices/soc.0/qcom,ara-codec.82/power/wakeup_active
1
Music Playback stopped
$ cat /sys/devices/soc.0/qcom,ara-codec.82/power/wakeup_active
0

Tested-by: David Lin <dtwlin@google.com>
Reviewed-by: David Lin <dtwlin@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Akash Choudhari <akashtc@google.com>
Signed-off-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>