bridge: Add ability to enable TSO

Currently a bridge device turns off TSO feature if no bridge ports
support it. We can always enable it, since packets can be segmented on
ports by software as well as on the bridge device.
This will reduce the number of packets processed in the bridge.

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>