ASoC: cs35l33: mark PM functions as __maybe_unused

The newly added cs35l33 driver produces a harmless warning when
CONFIG_PM is disabled:

sound/soc/codecs/cs35l33.c:908:12: error: 'cs35l33_runtime_suspend' defined but not used [-Werror=unused-function]
sound/soc/codecs/cs35l33.c:868:12: error: 'cs35l33_runtime_resume' defined but not used [-Werror=unused-function]

This adds __maybe_unused annotations to shut up the warning
regardless of the configuration.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Paul Handrigan <Paul.Handrigan@cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>