selftests: mlxsw: sch_red_ets: Increase required backlog

Backlog fluctuates on Spectrum-4 much more than on <4. Increasing the
desired backlog seems to help, as the constant fluctuations do not overlap
into the territory where packets are marked.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Link: https://patch.msgid.link/0821fb3aa8bb6a6c0d3000baab04995517c9a0cc.1728316370.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>