mlxsw: span: Remove span_entry by span_id

Instead of removing span_entry by the port number, allow removing by
SPAN id. That simplifies some code right here, and for mirroring to soft
netdevices, avoids problems with netdevice pointer invalidation and
reuse.

Rename mlxsw_sp_span_entry_find() to mlxsw_sp_span_entry_find_by_port()
and keep it--follow-up patches will make use of it.

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