x86, wmi fix modalias_show return values

I just fixed this same bug in arch/powerpc/kernel/vio.c and took a quick
look for other similar errors in the kernel.

modalias_show() should return an empty string on error, not errno.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Matthew Garrett <matthew.garrett@nebula.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>