bonding: improve event debug usability

Seeing bonding debug log data along the lines of "event: 5" is a bit spartan,
and often requires a lookup table if you don't remember what every event is.
Make use of netdev_cmd_to_name for an improved debugging experience, so for
the prior example, you'll see: "bond_netdev_event received NETDEV_REGISTER"
instead (both are prefixed with the device for which the event pertains).

CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Veaceslav Falico <vfalico@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: netdev@vger.kernel.org
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>