Fix neighbor discovery ethernet address saving

Fix neighbor discovery ethernet address saving

When a successful neighbor advertisement is received, the ethernet
address should be saved for later use to avoid having to redo the
neighbor discovery process.

For example, with TFTP the address should get saved into
"net_server_ethaddr".  This is being done correctly with ARP for IPv4,
but not for neighbor discovery with IPv6.

Signed-off-by: Sean Edmond <[email protected]>
Reviewed-by: Viacheslav Mitrofanov <[email protected]>