bonding: fire NETDEV_RELEASE event only on 0 slaves

Currently, if we set up netconsole over bonding and release a slave,
netconsole will stop logging on the whole bonding device. Change the
behavior to stop the netconsole only when the last slave is released.

Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>