libcxgbi:cxgb4i Guard ipv6 code with a config check

Fixes: fc8d0590 ("libcxgbi: Add ipv6 api to driver")
Fixes: 759a0cc5

 ("cxgb4i: Add ipv6 code to driver, call into libcxgbi ipv6
api")
Signed-off-by: Anish Bhatt <anish@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>