iio: gyro: bmg160: use regmap to retrieve struct device

Driver includes struct regmap and struct device in its global data.
Remove the struct device and use regmap API to retrieve device info.

Patch created using Coccinelle plus manual edits.
Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Reviewed-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>