staging:iio: Use module_i2c_driver to register I2C drivers

Use the newly introduced module_i2c_driver macro for registering I2C drivers.
This allows us to remove a few lines of boilerplate code.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>