ASoC: cx2072x: mark PM function as __maybe_unused

While the suspend function is already marked __maybe_unused,
the resume function is not, which leads to a warning when
CONFIG_PM is disabled:

sound/soc/codecs/cx2072x.c:1625:12: error: unused function 'cx2072x_runtime_resume' [-Werror,-Wunused-function]

Mark this one like the other one.

Fixes: a497a436 ("ASoC: Add support for Conexant CX2072X CODEC")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>