net: stmmac: Fix VLAN filter delete timeout issue in Intel mGBE SGMII

For Intel mGbE controller, MAC VLAN filter delete operation will time-out
if serdes power-down sequence happened first during driver remove() with
below message.

[82294.764958] intel-eth-pci 0000:00:1e.4 eth2: stmmac_dvr_remove: removing driver
[82294.778677] intel-eth-pci 0000:00:1e.4 eth2: Timeout accessing MAC_VLAN_Tag_Filter
[82294.779997] intel-eth-pci 0000:00:1e.4 eth2: failed to kill vid 0081/0
[82294.947053] intel-eth-pci 0000:00:1d.2 eth1: stmmac_dvr_remove: removing driver
[82295.002091] intel-eth-pci 0000:00:1d.1 eth0: stmmac_dvr_remove: removing driver

Therefore, we delay the serdes power-down to be after unregister_netdev()
which triggers the VLAN filter delete.

Fixes: b9663b7c ("net: stmmac: Enable SERDES power up/down sequence")
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>