iio: magn/ak8974: avoid unused function warning

iio: magn/ak8974: avoid unused function warning

The ak8974_configure() function is used only from the PM code,
but that can be hidden when CONFIG_PM is disabled:

drivers/iio/magnetometer/ak8974.c:201:12: error: 'ak8974_configure' defined but not used [-Werror=unused-function]

This replaces the #ifdef with a __maybe_unused annotation, which
will work correctly in all configurations and avoid the warning,
as the compiler can now see where ak8974_configure is called from.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 7c94a8b2ee8c ("iio: magn: add a driver for AK8974")
Signed-off-by: Jonathan Cameron <jic23@kernel.org>