wlan: Start idle scan timer on STA disconnection

On STA disconnection, idle scan timer is not started.
This will result in device not getting placed in IMPS
state and not allowing APPS power collapse.

Fix is to start idle scan scan timer on STA disconnection
so that device will be put in IMPS state.

Change-Id: I4e47f5b50866d503453e6a23688b2a4c406b251b
CRs-Fixed: 2287372