net: devmem: ksft: add exit_wait to make rx test pass

This exit_wait seems necessary to make the rx side test pass for me.
I think this is just missed from the original test add patch. Add it now.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250523230524.1107879-6-almasrymina@google.com


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