net: stmmac: disable/enable ptp_ref_clk in suspend/resume flow

disable ptp_ref_clk in suspend flow, and enable it in resume flow.

Fixes: f573c0b9 ("stmmac: move stmmac_clk, pclk, clk_ptp_ref and stmmac_rst to platform structure")
Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>