PCI: acpiphp: fix function 0 leak when disabling a slot

Previously, we acquired two references to function 0, but only released
one.

[bhelgaas: split this out from "remove all functions" fix]
Signed-off-by: Amos Kong <kongjianjun@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>