ipv6: remove unnecessary check in addrconf_prefix_rcv_add_addr()

Remove unnecessary check on update_lft variable in
addrconf_prefix_rcv_add_addr routine since it is always set to 0.
Moreover remove update_lft re-initialization to 0

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>