net: net_failover: fix typo in net_failover_slave_register()

Sync both unicast and multicast lists instead of unicast twice.

Fixes: cfc80d9a

 ("net: Introduce net_failover driver")
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>