selftests: drv-net: devmem: use new mattr ynl helpers

Use the just-added YNL helpers instead of manually setting
"_present" bits in the queue attrs. Compile tested only.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Acked-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20250723171046.4027470-6-kuba@kernel.org


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