qlcnic: track vxlan port count

The callback for adding vxlan port can be called with the same port for
both IPv4 and IPv6. Do not disable the offloading when the same port for
both protocols is added and later one of them removed.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>