PCI: dwc: Enable write permission for Class Code, Interrupt Pin updates

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