mlxsw: spectrum_span: Only update mirroring agents if present

In order not to needlessly schedule the work item that updates the
mirroring agents, only schedule it if there are any mirroring agents
present.

This is done by adding an atomic counter that counts the active
mirroring agents.

It is incremented / decremented whenever a mirroring agent is created /
destroyed. It is read before scheduling the work item and in the
devlink-resource occupancy callback.

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