iio: isl29018: Fix uninitialized value

The lux_uscale value is not initialized at probe. The value will be
uninitialized unless a value is written to it through the iio channel interface.
This fixes that.

Signed-off-by: Derek Basehore <dbasehore@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/65998


Signed-off-by: Jonathan Cameron <jic23@kernel.org>