acpi ec: Fix possible double io port registration

which will result in a harmless but ugly WARN message on
some machines.

Signed-off-by: Thomas Renninger <trenn@suse.de>
CC: mjg59@srcf.ucam.org
CC: platform-driver-x86@vger.kernel.org
CC: linux-acpi@vger.kernel.org
CC: astarikovskiy@suse.de
CC: akpm@linux-foundation.org
Signed-off-by: Matthew Garrett <mjg@redhat.com>