+- max-link-speed:
+ If present this property specifies PCI gen for link capability. The host drivers
+ could add this as a strategy to avoid unnecessary operation for unsupported
+ link speed, for instance, trying to do training for unsupported link speed, etc.
+ Must be '4' for gen4, '3' for gen3, '2' for gen2, and '1' for gen1. Any other
+ values are invalid.