iio:magn:mmc35240: Switch from CONFIG_PM_SLEEP guards to pm_sleep_ptr() etc

Letting the compiler remove these functions when the kernel is built
without CONFIG_PM_SLEEP support is simpler and less error prone than the
use of #ifdef based config guards. Also use SIMPLE_DEV_PM_OPS instead
of open-coding the equivalent.

Removing instances of this approach from IIO also stops them being
copied into new drivers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220130193147.279148-32-jic23@kernel.org