gdhcp: Don't send DHCPREQUEST if last assigned IP is Link Local Address

gdhcp: Don't send DHCPREQUEST if last assigned IP is Link Local Address

In the following scenario:
 1. ConnMan is connected to a service and Link Local Address is obtained.
 2. Disconnect the service.
 3. Connect the service again.
ConnMan tries to send DHCPREQUEST with last assigned IP which was Link
Local Address.

This patch makes ConnMan to send DHCPDISCOVER when last assigned IP
was Link Local Address.

Change-Id: I3c03f2b43a0f5c5f3be0a4878decb4345af55b33
Signed-off-by: Saurav Babu <saurav.babu@samsung.com>