mlxsw: spectrum_span: Allow passing parameters to SPAN agents

Currently, the only parameter of a SPAN agent is the netdev which
the SPAN agent should mirror to.

The next patch will add the ability to request a SPAN agent that mirrors
to a specific netdev and has a specific policer ID bound to it. This is
required when mirroring packets to the CPU port.

Therefore, encapsulate the sole parameter to mlxsw_sp_span_agent_get()
in a structure, so that it could later be extended with policer
information.

Reviewed-by: Jiri Pirko <jiri@mellanox.com>
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>