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>