Tx/Rx clock speed. This speed selection is valid when either
internal or external clocking is selected. In some cases, router
software uses this attribute for route selection. If Priority
Queuing is configured, it chooses queue depths based on this
attribute. If internal clocking is selected, this attribute
effects hardware initialization. If external
clocking is selected, this attribute should be set to the speed that
most closely corresponds to the speed of the external clock.