qcacld-3.0: report correct max tx rate

1. NSS from FW is not reliable as it changes as per the environment
   quality, so take static NSS for fetching max tx rate;
2. Fix the MCS index to 0 if it's invalid when fetching max tx rate;
3. Remove useless code in hdd_report_max_rates().

CRs-Fixed: 2777241
Change-Id: I065748b59feba6a61859fe58bb6429d96da50441