mlxsw: spectrum_switchdev: Handle SWITCHDEV_PORT_ATTR_SET

Following patches will change the way we communicate setting a port's
attribute and use a notifier to perform those tasks.

Prepare mlxsw to support receiving notifier events targeting
SWITCHDEV_PORT_ATTR_SET and utilize the switchdev_handle_port_attr_set()
to handle stacking of devices.

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>