if_re: Generate an address if there is none in the EEPROM

There exists hardware that has no ethernet address burned into
the EEPROM. Loading if_re on such a HW brings the device up
with '00:00:00:00:00:00' as the address, and that doesn't get
you too far in a real network.

PR: 262406
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/670
Signed-off-by: Evgeni Golov <evgeni@debian.org>
Differential Revision: https://reviews.freebsd.org/D34485