net: ethernet: dwmac: fix non static symbol warning

Fixes the following sparse warning:

drivers/net/ethernet/stmicro/stmmac/dwmac-stm32.c:172:1: warning:
 symbol 'stm32_dwmac_pm_ops' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/dwmac-stm32.c

@@ -169,7 +169,8 @@ static int stm32_dwmac_resume(struct device *dev)
 }
 #endif /* CONFIG_PM_SLEEP */
 
-SIMPLE_DEV_PM_OPS(stm32_dwmac_pm_ops, stm32_dwmac_suspend, stm32_dwmac_resume);
+static SIMPLE_DEV_PM_OPS(stm32_dwmac_pm_ops,
+	stm32_dwmac_suspend, stm32_dwmac_resume);
 
 static const struct of_device_id stm32_dwmac_match[] = {
 	{ .compatible = "st,stm32-dwmac"},