PCI: artpec6: Stop enabling writes to DBI read-only registers

Previously we enabled writes to the DBI read-only registers so the Class
Code fix in dw_pcie_setup_rc() would work.  But now dw_pcie_setup_rc()
enables write permission itself, so we don't need to do it here.

Stop enabling writes to the DBI read-only registers.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Roy Zang <tie-fei.zang@freescale.com>