hsr: remove hsr interface if all slaves are removed

When all hsr slave interfaces are removed, hsr interface doesn't work.
At that moment, it's fine to remove an unused hsr interface automatically
for saving resources.
That's a common behavior of virtual interfaces.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>