wlan: Clean scan cache for connected AP when disconnection

When Dut connected with a target AP, if power off AP,
there is still chance wpa_supplicant choose the target
ap to connect, as the scan cache which was stored before
power off will be reported to userspace.

Clean up scan cache for target AP when disconnection
happens.

And this is a regression issue, regression cause is
I850fd336fc73e4b2c2362dfd11db433b68ccd1fb.

Change-Id: I802cb7d0996592705681f145f00196cad1336df7
CRs-Fixed: 2330607