PCI: layerscape: Modify the way of getting capability with different PEX

PCI: layerscape: Modify the way of getting capability with different PEX

The different PCIe controller in one board may be have different
capability of MSI or MSIX, so change the way of getting the MSI
capability, make it more flexible.

Link: https://lore.kernel.org/r/20200918080024.13639-8-Zhiqiang.Hou@nxp.com
Signed-off-by: Xiaowei Bao <xiaowei.bao@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>