if_edsc: generate an arbitrary MAC address

When generating an cloned interface instance in edsc_clone_create(),
generate a MAC address from the FF OUI with ether_gen_addr(). This allows us
to have unique local-link addresses. Previously, the MAC address was zero.

Submitted by: Neel Chauhan <neel AT neelc DOT org>
Differential Revision: https://reviews.freebsd.org/D23842