net: stmmac: Fix an error code in probe()

The function should return an error if create_singlethread_workqueue()
fails.

Fixes: 34877a15 ("net: stmmac: Rework and fix TX Timeout code")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>