x86, efi: Fix argument types for SetVariable()

The spec says this takes uint32 for attributes, not uintn.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Link: http://lkml.kernel.org/r/1307388985-7852-1-git-send-email-mjg@redhat.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>