iio: bmg160: Remove i2c_client from data struct

i2c_client variable is not really used anymore in the core driver. It is
only used to get the device to make proper outputs.

This patch replaces all i2c_client usage through direct usage of the
device pointer.

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>