bnx2x: Fix vxlan removal

Commmit ac7eccd4 "bnx2x: track vxlan port count" contains a bug -
Instead of achieving the required goal, vxlan configuration would not
be removed since we're decrementing the port instead of the counter.

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