selftests: drv-net: Fix and clarify TC bandwidth split in devlink_rate_tc_bw.py

selftests: drv-net: Fix and clarify TC bandwidth split in devlink_rate_tc_bw.py

Correct the documented bandwidth distribution between TC3 and TC4
from 80/20 to 20/80. Update test descriptions and printed messages
to consistently reflect the intended split.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Nimrod Oren <noren@nvidia.com>
Link: https://patch.msgid.link/20251130091938.4109055-6-cjubran@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>