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>

Author: mina

tools/testing/selftests/drivers/net/hw/devmem.py

@@ -26,7 +26,7 @@ def check_rx(cfg) -> None:
     port = rand_port()
     listen_cmd = f"{cfg.bin_local} -l -f {cfg.ifname} -s {cfg.addr} -p {port}"
 
-    with bkg(listen_cmd) as ncdevmem:
+    with bkg(listen_cmd, exit_wait=True) as ncdevmem:
         wait_port_listen(port)
         cmd(f"echo -e \"hello\\nworld\"| socat -u - TCP{cfg.addr_ipver}:{cfg.addr}:{port}", host=cfg.remote, shell=True)