qcacld-3.0: Add API to get max mcs index

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

core/sap/inc/sap_api.h

@@ -275,6 +275,7 @@ typedef struct sap_StationAssocReassocCompleteEvent_s {
 	uint8_t max_supp_idx;
 	uint8_t max_ext_idx;
 	uint8_t max_mcs_idx;
+	uint8_t max_real_mcs_idx;
 	uint8_t rx_mcs_map;
 	uint8_t tx_mcs_map;
 	uint8_t ecsa_capable;