Add a new field max_mcs_index in mlme_legacy_priv to save max mcs
index of current vdev.
Add a new field max_real_mcs_idx in hdd_station_info to save max
mcs index of the connected station.
Change-Id: I28908515cbe5c18c79f14f8645defd5c82e3a6f0
CRs-Fixed: 3065838