ASoC: rt711: Add __maybe_unused to PM callbacks

ASoC: rt711: Add __maybe_unused to PM callbacks

Fix the compile warnings by adding __maybe_unused to PM callbacks:
  sound/soc/codecs/rt711-sdw.c:504:12: warning: ‘rt711_dev_resume’ defined but not used [-Wunused-function]
  sound/soc/codecs/rt711-sdw.c:490:12: warning: ‘rt711_dev_suspend’ defined but not used [-Wunused-function]

Fixes: c08460627e3b ("ASoC: rt711: add rt711 codec driver")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20200127192831.14057-4-tiwai@suse.de
Signed-off-by: Mark Brown <broonie@kernel.org>