ALSA: cs5535audio: fix unused warnings on resume/suspend

When CONFIG_PM_SLEEP is disabled, SIMPLE_DEV_PM_OPS does not use
snd_cs5535audio_resume and snd_cs5535audio_suspend functions:

sound/pci/cs5535audio/cs5535audio_pm.c:77:12: warning: ‘snd_cs5535audio_resume’ defined but not used [-Wunused-function]
 static int snd_cs5535audio_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~~~
sound/pci/cs5535audio/cs5535audio_pm.c:58:12: warning: ‘snd_cs5535audio_suspend’ defined but not used [-Wunused-function]
 static int snd_cs5535audio_suspend(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~~~~

Adding __maybe_unused to the declaration of these functions removes the
warnings.

Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Signed-off-by: Takashi Iwai <tiwai@suse.de>