libcxgbi/cxgb4i : Fix ipv6 build failure caught with randconfig

Previous guard of IS_ENABLED(CONFIG_IPV6) is not sufficient when cxgbi drivers
are built into kernel but ipv6 is not.

v2: Use Kconfig to disable compiling cxgbi built into kernel when ipv6 is
compiled as a module

Fixes: e81fbf6cd652 ("libcxgbi:cxgb4i Guard ipv6 code with a config check")
Fixes: fc8d0590d914 ("libcxgbi: Add ipv6 api to driver")
Signed-off-by: Anish Bhatt <anish@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>