mlxsw: spectrum: Drop struct span_entry.used

The member ref_count already determines whether a given SPAN entry is
used, and is as easy to use as a dedicated boolean.

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