mlxsw: spectrum: Register SPAN before switchdev

Since switchdev events can trigger SPAN respin, it is necessary that the
data structures are available. Register SPAN first, with a commentary on
what the dependencies are.

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