[BRIDGE]: Assign random address.

Assigning a valid random address to bridge device solves problems
when bridge device is brought up before adding real device to bridge.
When the first real device is added to the bridge, it's address
will overide the bridges random address.

Note: any device added to a bridge must already have a valid
ethernet address.
 br_add_if -> br_fdb_insert -> fdb_insert -> is_valid_ether_addr

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>