qcacmn: Roam control configuration for 6 GHz in full scan only on prior discovery

During the roam scan, if there are no desired APs found in the partial
frequency list, an immediate full scan on all the supported frequencies
is initiated as a fallback. This would include the 6 GHz PSC
frequencies. Define an attribute to allow that behavior to be modified
to include PSCs only if 6 GHz use has been detected.

Change-Id: I81eb9b20c2953e6b0dbf4138b31547db82d7397f
CRs-Fixed: 3351706