PCI: dwc: Split MSI IRQ parsing/allocation to a separate function

Split handling of MSI host IRQs to a separate dw_pcie_msi_host_init()
function. The code is complex enough to warrant a separate function.

[bhelgaas: reorder patch earlier]
Link: https://lore.kernel.org/r/20220707134733.2436629-4-dmitry.baryshkov@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>