ALSA: x86: mark hdmi suspend/resume functions as __maybe_unused

The two functions are unused when CONFIG_PM_SLEEP is disabled:

sound/x86/intel_hdmi_audio.c:1633:12: error: 'hdmi_lpe_audio_resume' defined but not used [-Werror=unused-function]
sound/x86/intel_hdmi_audio.c:1622:12: error: 'hdmi_lpe_audio_suspend' defined but not used [-Werror=unused-function]

Marking them as __maybe_unused avoids the warning without introducing an
ugly #ifdef.

Fixes: 182cdf23 ("ALSA: x86: Implement runtime PM")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>