staging: iio: isl29018: use regmap to retrieve struct device

Remove struct device from drivers global data and use regmap
API to retrieve device info instead.

This replacement can be done for drivers that include regmap
in their global data.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>