mlxsw: spectrum_span: On policer_id_base_ref_count, use dec_and_test

When unsetting policer base, the SPAN code currently uses refcount_dec().
However that function splats when the counter reaches zero, because
reaching zero without actually testing is in general indicative of a
missing cleanup. There is no cleanup to be done here, but nonetheless, use
refcount_dec_and_test() as required.

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