[BRIDGE]: allow setting hardware address of bridge pseudo-dev

Some people are using bridging to hide multiple machines from an ISP
that restricts by MAC address. So in that case allow the bridge mac
address to be set to any of the existing interfaces.  I don't want to
allow any arbitrary value and confuse STP.

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