iio: bcm150: Remove i2c_client from private data

i2c_client struct is now only used for debugging output. We can use the
device struct as well so we can remove all struct i2c_client usage.

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