PCI: artpec6: Drop __init from artpec6_add_pcie_port()

artpec6_add_pcie_port() is called from artpec6_pcie_probe(), which is not
marked __init.  It is wrong to call an __init function from a non-__init
one, so remove __init from artpec6_add_pcie_port().

[bhelgaas: changelog]
Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>