mlxsw: spectrum_router: Fix rif counter freeing routine

mlxsw: spectrum_router: Fix rif counter freeing routine

During rif counter freeing the counter index can be invalid. Add check
of validity before freeing the counter.

Fixes: e0c0afd8aa4e ("mlxsw: spectrum: Support for counters on router interfaces")
Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>