cfg80211: Add support to sched scan to report better BSSs

Enhance sched scan to support option of finding a better BSS while in
connected state. Firmware scans the medium and reports when it finds a
known BSS which has better RSSI than the current connected BSS. New
attributes to specify the relative RSSI (compared to the current BSS)
are added to the sched scan to implement this.

Signed-off-by: vamsi krishna <vamsin@qti.qualcomm.com>
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>