staging: iio: isl29028: use the runtime power management for system sleep

With the introduction of runtime power management in commit 2db5054a
("staging: iio: isl29028: add runtime power management support"), the
system could go to sleep and turn off the device without notifying the
runtime power management code. This patch changes the system suspend
and resume to go through the runtime power management.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>