sgiioc4: Disable module unload

This patch removes a module_exit function that sgiioc4 should not have had.

It seems that the IDE layer doesn't support submodule unloading. sgiioc4
was the only driver in drivers/ide/pci that had an exit function.
After an unload, the devices would stay around and the next attempt to
reference would crash...

Signed-off-by: Jeremy Higdon <jeremy@sgi.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>