qcacld-3.0: Introduce ini to configure roam RSSI diff for 6 GHz AP

This is applicable only when the roaming happens from 2.4 GHz/5 GHz
to 6 GHz AP.
The value is used to check if the 6 GHz AP to which we are roaming
(from 2.4 GHz/ 5 GHz) is better than current AP in terms of RSSI.
This checking is disabled if the value is set to zero.

Change-Id: I77f1183e1fd704dfa153841e669a20c9e75d0fe7
CRs-Fixed: 3353346