iio: health/afe4404: mark suspend/resume functions __maybe_unused

The newly added afe4404 driver implements suspend/resume using the
SIMPLE_DEV_PM_OPS() macro, which leaves out references to the actual
functions when CONFIG_PM is disabled, causing a harmless warning:

health/afe4404.c:509:12: error: 'afe4404_suspend' defined but not used
health/afe4404.c:530:12: error: 'afe4404_resume' defined but not used

This marks the functions as __maybe_unused so we don't get those
warnings.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 87aec56e ("iio: health: Add driver for the TI AFE4404 heart monitor")
Signed-off-by: Jonathan Cameron <jic23@kernel.org>