qcacmn: Define QCA vendor attribute for high RSSI roam trigger threshold

Define an attribute for high RSSI roam trigger threshold. STA is
expected to trigger roam if the current connected AP's RSSI gets above
this high RSSI threshold. STA's roam attempt on high RSSI threshold aims
to find candidates from other better Wi-Fi bands.

Change-Id: Ie0323ff990e2679fd9768d2242730f7369cdb64f
CRs-Fixed: 3586127