selftests: mlxsw: tc_restrictions: Fix test failure with new iproute2

selftests: mlxsw: tc_restrictions: Fix test failure with new iproute2

As explained in [1], iproute2 started rejecting tc-police burst sizes
that result in an overflow. This can happen when the burst size is high
enough and the rate is low enough.

A couple of test cases specify such configurations, resulting in
iproute2 errors and test failure.

Fix by reducing the burst size so that the test will pass with both new
and old iproute2 versions.

[1] https://lore.kernel.org/netdev/20250916215731.3431465-1-jay.vosburgh@canonical.com/

Fixes: cb12d1763267 ("selftests: mlxsw: tc_restrictions: Test tc-police restrictions")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/88b00c6e85188aa6a065dc240206119b328c46e1.1770643998.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>