e1000/base: fix link flap on 82579

e1000/base: fix link flap on 82579

Several customers have reported a link flap issue on 82579. The symptoms
are random and intermittent link losses when 82579 is connected to specific
switches. Issue has been root caused as interoperability problem between
the NIC and at least some Broadcom PHYs in the Energy Efficient Ethernet
wake mechanism.
To fix the issue, we are disabling the Phase Locked Loop shutdown in 100M
Low Power Idle. This solution will cause an increase of power in 100M EEE
link. It may cost additional 28mW in this specific mode.

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>