mlxsw: spectrum_switchdev: Do not assume notifier information type

VxLAN notifications are going to use a different notifier information
type, so cast to the correct type based on the received event.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>