switchdev: Add SWITCHDEV_PORT_ATTR_SET

In preparation for allowing switchdev enabled drivers to veto specific
attribute settings from within the context of the caller, introduce a
new switchdev notifier type for port attributes.

Suggested-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>