Change to check low ID register to determine the valid PHY address,
for yt8521s PHY with high ID register value 0. And fix polling
register when expect value is 0, to complete MDIO read.
Fixes: 44e97550ca ("net/ngbe: identify and reset PHY")
Cc: stable@dpdk.org
Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>