bonding: check for assigned mac before adopting the slaves mac address

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>