[BRIDGE]: set features based on enslaved devices

Make features of the bridge pseudo-device be a subset of the underlying
devices.  Motivated by Xen and others who use bridging to do failover.

Signed-off-by: Catalin BOIE <catab at umrella.ro>
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>