bridge: add new hwmode swdev

Current hwmode settings are "vepa" or "veb".  These are for NIC interfaces
with basic bridging function offloaded to HW.  Add new "swdev" for full
switch device offloads.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>