dw_pcie_setup_rc() contains fixes to update the Class Code and Interrupt
Pin registers, but the fixes don't actually work because these registers
are read-only.

Enable write permission before updating the Class Code and Interrupt
Pin.
Signed-off-by: NHou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Acked-by: NJoao Pinto <jpinto@synopsys.com>
Acked-by: NRoy Zang <tie-fei.zang@freescale.com>