iio: isl29018: Support suspend and resume.

The driver leaves the device in power-down state anyway,
so there is nothing to do on suspend.
On resume, we just have to make sure the range and ADC
values are updated in the device since it may have been
powered down in suspend.

Signed-off-by: Bryan Freed <bfreed@chromium.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>