ALSA: interwave: guard PM-only restore helpers with CONFIG_PM

ALSA: interwave: guard PM-only restore helpers with CONFIG_PM

The InterWave PM patch added snd_interwave_restore_regs() and
snd_interwave_restore_memory() as static helpers, but both are used only
from the resume path under CONFIG_PM.

On configurations without CONFIG_PM, such as alpha allyesconfig, this
leaves both helpers unused and triggers -Wunused-function warnings with
W=1.

Move the PM-only helpers into the existing CONFIG_PM section. Keep
__snd_interwave_restore_regs() outside the guard because it is also used
during probe-time initialization.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202604101958.x16oNkfo-lkp@intel.com/
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260410-alsa-interwave-pm-warning-fix-v1-1-434d14c9c262@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>