[BRIDGE]: recompute features when adding a new device

We must recompute bridge features everytime the list of underlying 
devices changes, or we might end up with features that are not
supported by all devices (eg. NETIF_F_TSO)
This patch adds the missing recompute when adding a device to the bridge.

Signed-off-by: Olaf Rempel <razzor@kopf-tisch.de>
Signed-off-by: David S. Miller <davem@davemloft.net>