ALSA: hda/realtek: Fix build error without CONFIG_PM

ALSA: hda/realtek: Fix build error without CONFIG_PM

The alc_spec.power_hook is defined only with CONFIG_PM, and the recent
fix overlooked it, resulting in a build error without CONFIG_PM.
Fix it with the simple ifdef and set __maybe_unused for the function.

We may drop the whole CONFIG_PM dependency there, but it should be
done in a separate cleanup patch later.

Fixes: 1e707769df07 ("ALSA: hda/realtek - Set GPIO3 to default at S4 state for Thinkpad with ALC1318")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202405012104.Dr7h318W-lkp@intel.com/
Message-ID: <20240502062442.30545-1-tiwai@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>