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: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>