Thank you for the feedback. After applying (only) the suggested change, testing a EEE state change with my hardware looks like the following:
root@freebsd:/usr/src # sysctl dev.ix.1.eee_state dev.ix.1.eee_state: 1 root@freebsd:/usr/src # sysctl dev.ix.1.eee_state=0 dev.ix.1.eee_staix1: Error in EEE setup: 0x7FFFFFFF te: 1 sysctl: dev.ix.1.eee_state=0: Invalid argument root@freebsd:/usr/src #