mlxsw: spectrum_span: Do no expose mirroring agents to entire driver

The struct holding the different mirroring agents is currently allocated
as part of the main driver struct. This is unlike other driver modules.

Allocate the memory required to store the different mirroring agents as
part of the initialization of the mirroring module.

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