selftests: drv-net: rss_ctx: make the test pass with few queues

rss_ctx.test_rss_key_indir implicitly expects at least 5 queues,
as it checks that the traffic on first 2 queues is lower than
the remaining queues when we use all queues. Special case fewer
queues.

Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250901173139.881070-2-kuba@kernel.org


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