cxgb4i : Call into recently added cxgb4 ipv6 api

Get a reference on every ipv6 address we offload to hardware so that it cannot
be prematurely cleared out before cleanup.

Signed-off-by: Anish Bhatt <anish@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>