Wi-Fi: Set BTCOEXMODE_DISABLED irrespective of BT's connection state

BTCOEXMODE_DISABLE has to be triggered during the DHCP phase even
on an active BT connection to ensure that Wi-Fi is given
preference over the BT.
This commit ensures the same
CRs-Fixed: 695307

Change-Id: I1eb979f03696831cb8f9e3a14b8a62f724347b64