ASoC: rt1308-sdw: Add __maybe_unused to PM callbacks

Fix the compile warnings by adding __maybe_unused to PM callbacks:
  sound/soc/codecs/rt1308-sdw.c:690:12: warning: ‘rt1308_dev_resume’ defined but not used [-Wunused-function]
  sound/soc/codecs/rt1308-sdw.c:676:12: warning: ‘rt1308_dev_suspend’ defined but not used [-Wunused-function]

Fixes: a87a6653 ("ASoC: rt1308-sdw: add rt1308 SdW amplifier driver")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20200127192831.14057-2-tiwai@suse.deSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/rt1308-sdw.c

@@ -673,7 +673,7 @@ static const struct sdw_device_id rt1308_id[] = {
 };
 MODULE_DEVICE_TABLE(sdw, rt1308_id);
 
-static int rt1308_dev_suspend(struct device *dev)
+static int __maybe_unused rt1308_dev_suspend(struct device *dev)
 {
 	struct rt1308_sdw_priv *rt1308 = dev_get_drvdata(dev);
 
@@ -687,7 +687,7 @@ static int rt1308_dev_suspend(struct device *dev)
 
 #define RT1308_PROBE_TIMEOUT 2000
 
-static int rt1308_dev_resume(struct device *dev)
+static int __maybe_unused rt1308_dev_resume(struct device *dev)
 {
 	struct sdw_slave *slave = dev_to_sdw_dev(dev);
 	struct rt1308_sdw_priv *rt1308 = dev_get_drvdata(dev);