pci_bridge_dev: fix error path in pci_bridge_dev_initfn()

Currently, we do not properly cleanup, if pci_bridge_dev_initfn
fails to initialize properly. Make sure to call pci_bridge_exitfn()
in the error path.

Signed-off-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>