nl80211: specify RSSI threshold in scheduled scan

Support configuring an RSSI threshold in dBm (s32) when requesting
scheduled scan, below which a BSS won't be reported by the cfg80211
driver.

Signed-off-by: Thomas Pedersen <c_tpeder@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>