cxgb4 ipv6 does not guard against ipv6 being disabled, or the standard
ipv6 module vs inbuilt tri-state issue. This was fixed for cxgb4i & iw_cxgb4
but missed for cxgb4.
Signed-off-by: NAnish Bhatt <anish@chelsio.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>