iio: bmg160: Use generic dev_drvdata

i2c_get_clientdata() is specifically for i2c. Replace it with the
generic dev_get/set_drvdata() to support different protocols.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Reviewed-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>