switchdev: add new switchdev_port_bridge_getlink

Like bridge_setlink, add switchdev wrapper to handle bridge_getlink and
call into port driver to get port attrs.  For now, only BR_LEARNING and
BR_LEARNING_SYNC are returned.  To add more, we'll probably want to break
away from ndo_dflt_bridge_getlink() and build the netlink skb directly in
the switchdev code.

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