stmmac: dwmac-socfpga: make socfpga_dwmac_pm_ops static

Fix the following sparse warning:
drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c:274:1: warning:
  symbol 'socfpga_dwmac_pm_ops' was not declared. Should it be static?

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>