PCI: Drop redundant setting of bus->is_added in virtfn_add_bus()

The flag pci_bus->is_added is used to guard invocation of
pcibios_fixup_bus(pci_bus).  When virtfn_add_bus() is called, the
pci_bus->is_added flag has already been set, so remove the redundant

	bus->is_added = 1;

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Donald Dutile <ddutile@redhat.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Ram Pai <linuxram@us.ibm.com>