Move lle_event to if_llatbl.h

lle_event replaced arp_update_event after the ARP rewrite and ended up
in if_ether.h simply because arp_update_event used to be there too.
IPv6 neighbor discovery is going to grow lle_event support and this is a
good time to move it to if_llatbl.h.

The two in-tree consumers of this event - OFED and toecore - are not
affected.

Reviewed by: bz@