External PHY cannot power down after LAN reset, so need to manually
power down when device stopped.
Fixes: 3d0af70667 ("net/ngbe: setup PHY link")
Fixes: 1c44384fce ("net/ngbe: support custom PHY interfaces")
Cc: stable@dpdk.org
Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>