PCI: rockchip: Factor out rockchip_pcie_setup_irq()

Factor out rockchip_pcie_setup_irq() to prepare for future bug fixes. No
functional change intended.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>