wlan: Abort existing scan before starting DHCP phase.

In case of BT coex, WLAN connection taking longer time, because of
ongoing scan happening during DHCP phase.
Aborting ongoing scan before sending DHCP START event to firmware.

Change-Id: I02b163d937ac52984bde571787c079778103160f
CRs-Fixed: 720672
Wlan: Revision 3.2.3.418

Revision 3.2.3.418

Change-Id: Ia57b097332a9c255ff1948351ecce7907d6a9bef
CRs-Fixed: 688141