selftests: mlxsw: Decrease required rate accuracy

On Spectrum-{2,3} the required accuracy is +/-10%.

Align the test to this requirement so that it can reliably pass on these
platforms.

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