mlxsw: spectrum: Convert callers to use new mirroring API

Previous patch added a work item in the mirroring code that will take
care of updating the active mirroring agents in response to different
events.

Change the mirroring agents update function - mlxsw_sp_span_respin() -
to invoke this work item when called.

Therefore there is no need for callers to schedule a work item
themselves.

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