cxgb4i : Fix -Wunused-function warning

A bunch of ipv6 related code is left on by default. While this causes no
compilation issues, there is no need to have this enabled by default. Guard
with an ipv6 check, which also takes care of a -Wunused-function warning.

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