selftests: mlxsw: Add Spectrum-2 target scale for tc flower scale test

Return the maximum number of tc flower filters that can be offloaded.
Currently, this value corresponds to the number of counters supported by
the driver.

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