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 c20811a79e671a6a1fe86a8c1afe04aca8a7f085
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>