iio: bmc150: Use i2c regmap

This replaces all usage of direct i2c accesses with regmap accesses.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Tested-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>