i40iw: Use runtime check for IS_ENABLED(CONFIG_IPV6)

To be consistent, use the runtime check instead of
conditional compile.

Signed-off-by: Mustafa Ismail <mustafa.ismail@intel.com>
Signed-off-by: Henry Orosco <henry.orosco@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>