net: stmmac: Move device_can_wakeup() check earlier in set_wol

If !device_can_wakeup(), there's no need to futher check. And return
-EOPNOTSUPP rather than -EINVAL if !device_can_wakeup().

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: David S. Miller <davem@davemloft.net>