selftests: mlxsw: Increase burst size for rate test

The current combination of rate and burst size does not adhere to
Spectrum-{2,3} limitation which states that the minimum burst size
should be 40% of the rate.

Increase the burst size in order to honor above mentioned limitation and
avoid intermittent failures of this test case on Spectrum-{2,3}.

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