staging: iio: hmc5843: Export missing SPI module alias information

The SPI core always reports the MODALIAS uevent as "spi:<modalias>"
regardless of the mechanism that was used to register the device
(i.e: OF or board code) and the table that is used later to match
the driver with the device (i.e: SPI id table or OF match table).

So drivers needs to export the SPI id table and this be built into
the module or udev won't have the necessary information to autoload
the needed driver module when the device is added.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>