ASoC: nau8825: mark pm functions __maybe_unused

The newly added nau8825_dai_is_active() function is only called from
the PM logic that is build-time conditional in this driver, so we get
a warning when CONFIG_PM is disabled:

sound/soc/codecs/nau8825.c:229:13: error: 'nau8825_dai_is_active' defined but not used [-Werror=unused-function]
 static bool nau8825_dai_is_active(struct nau8825 *nau8825)

By replacing the #ifdef around the functions with a __maybe_unused
annotation, the code becomes more robust to this kind of problem
and we no longer get the warning while also slightly improving
readability.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: b50455fa ("ASoC: nau8825: cross talk suppression measurement function")
Signed-off-by: Mark Brown <broonie@kernel.org>