iio:light:isl29018: Switch from CONFIG_PM_SLEEP guards to pm_sleep_ptr()

iio:light:isl29018: Switch from CONFIG_PM_SLEEP guards to pm_sleep_ptr()

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.

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

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Anson Huang <anson.huang@nxp.com>
Cc: Brian Masney <masneyb@onstation.org>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220130193147.279148-21-jic23@kernel.org