PCI: layerscape: Use dw_pcie_link_up() consistently

All the other DesignWare-based drivers use dw_pcie_link_up(), so use it in
this driver, too, for consistency.  No functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Pratyush Anand <pratyush.anand@gmail.com>