PCI: mediatek-gen3: Print LTSSM state when PCIe link down

Print current LTSSM state when PCIe link down instead of the register
value to make it easier to get the link status.

Link: https://lore.kernel.org/r/20220329030715.7975-1-jianjun.wang@mediatek.com


Signed-off-by: Jianjun Wang <jianjun.wang@mediatek.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>