Restore the check for an unassigned mac address before adopting the
first slaves as it's own. The change in behavior was introduced by:

commit c20811a7
Author: Jiri Pirko <jpirko@redhat.com>

    bonding: move dev_addr cpy to bond_enslave
Signed-off-by: David Strand <dpstrand@gmail.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>