net: encx24j600: Export missing SPI module alias information

The driver Kconfig symbol is tristate which means that it can be built as
a module but the module alias information is not added to the module info
so module autoload won't work since user-space won't have the information.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>