selftests: drv-net: devmem: flip the direction of Tx tests

selftests: drv-net: devmem: flip the direction of Tx tests

The Device Under Test should always be the local system.
While the Rx test gets this right the Tx test is sending
from remote to local. So Tx of DMABUF memory happens on remote.

These tests never run in NIPA since we don't have a compatible
device so we haven't caught this.

Reviewed-by: Joe Damato <joe@dama.to>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250811231334.561137-6-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>