staging:iio: Use module_spi_driver to register SPI driver

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

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