Add support to sched scan to report relatively better BSSs

Add support to sched scan to report relatively better BSSs

Add support to set sched scan relative RSSI parameters so that the
drivers can report BSSs after relative comparision with the current
connected BSS. This feature is applicable only when in connected mode.

The below commands can be used to configure relative RSSI parameters
SET relative_rssi <disable|rssi_value>
	disable - to disable the feature
	rssi_value - amount of relative RSSI in dB
SET relative_band_adjust <band:adjust_value>
	band - "2G" or "5G" for 2.4 GHz or 5 GHz respectively
	adjust_value - amount of RSSI to be adjusted in dB

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>