selftests: drv-net: rss_ctx: fix cleanup in the basic test

The basic test may fail without resetting the RSS indir table.
Use the .exec() method to run cleanup early since we re-test
with traffic that returning to default state works.
While at it reformat the doc a tiny bit.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240708213627.226025-2-kuba@kernel.org


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