net: stmmac: fix errno when create_singlethread_workqueue() fails

We should set the return value to -ENOMEM explicitly when
create_singlethread_workqueue() fails in stmmac_dvr_probe(),
otherwise we'll lose the error value.

Fixes: a137f3f2 ("net: stmmac: fix possible memory leak in stmmac_dvr_probe()")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20221214080117.3514615-1-cuigaosheng1@huawei.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>