macvlan: simplify the structure port

The port->count was used to count the number of macvlan devs
in the same port, but the list vlans could play the same role
to do that, so free the port if the list vlans is empty and
no need to use the parameter count.

Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>