e1000: prevent ULP flow if cable connected

Enabling ulp on link down when cable is connect caused an infinite
loop of linkup/down indications in the NDIS driver.
After discussed, correct flow is to enable ULP only when cable is
disconnected.

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

Approved by: imp
Obtained from: DPDK (4bff263d54d299269966365f9697941eecaa241b)
MFC after: 1 week