ASoC: rt1308-sdw: Add __maybe_unused to PM callbacks

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: a87a6653a28c ("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.de
Signed-off-by: Mark Brown <broonie@kernel.org>