cxgb4 : Fix build failure in cxgb4 when ipv6 is disabled/not in-built

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