PCI: release temporary reference in __nv_msi_ht_cap_quirk()

__nv_msi_ht_cap_quirk() acquires a temporary reference via
'pci_get_bus_and_slot()' that is never released.

This patch releases the temporary reference.

Signed-off-by: Myron Stowe <myron.stowe@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>