bonding: don't validate address at device open

The standard validate_addr handler refuses to accept the all zeroes address
as valid.  However, it's common historical practice for the bonding
master to be configured up prior to having any slaves, at which time the
master will have a MAC address of all zeroes.

Resolved by setting the dev->validate_addr to NULL.  The master still can't
end up with an invalid address, as the set_mac_address function tests
for validity.

Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>