bond: have random dev address by default instead of zeroes

Makes more sense to have randomly generated address by default than to
have all zeroes. It also allows user to for example put the bond into
bridge without need to have any slaves in it.

Also note that this changes only behaviour of bonds with no slaves. Once
the first slave device is enslaved, its address will be used (no change
here).

Also, fix dev_assign_type values on the way.

Reported-by: Pavel Šimerda <psimerda@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>