intel_sst: fix output noises when it's not in playback

When the corresponding output device is not in playback, we can hear a little
noises.

Fix it by powering on the device only when it's in playback.

Signed-off-by: Lu Guanqun <guanqun.lu@intel.com>
Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>