EthernetDataTracker: Start DHCP on Link Up

The behavior of mLinkUp has previously changed so that it only
represents the link state.  This changes fixes
interfaceLinkStateChanged() so that it triggers a reconnect based
only on the present link state.

Change-Id: I950e04e1f5b5019d59d3b9530f369f8b8f13134a
Signed-off-by: Adam Hampson <ahampson@google.com>