ipv6: ndisc: remove redundant check for !dev->addr_len

send_sllao is already initialized with the value of dev->addr_len

Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>