qcacld-3.0: Provide SNR value as SIGNAL_INFO to userspace for SAP

Currently, userspace is expecting SNR value through
NL80211_STA_INFO_SIGNAL for a disconnected client of SAP. But
the driver sends RSSI value by doing noise floor to the SNR value
received from firmware. Convert this back to SNR and provide to
userspace as per the definition of NL80211_STA_INFO_SIGNAL 
and to be in sync with the existing implementation of userspace.

Change-Id: Ia360424a9d44e48fae5a85cf0d2c577e92cd93be
CRs-Fixed: 2552542

core/hdd/inc/wlan_hdd_main.h

@@ -403,6 +403,8 @@ enum hdd_driver_flags {
 
 #define NUM_TX_RX_HISTOGRAM_MASK (NUM_TX_RX_HISTOGRAM - 1)
 
+#define HDD_NOISE_FLOOR_DBM (-96)
+
 /**
  * enum hdd_auth_key_mgmt - auth key mgmt protocols
  * @HDD_AUTH_KEY_MGMT_802_1X: 802.1x

core/hdd/src/wlan_hdd_station_info.c

@@ -821,7 +821,8 @@ static int32_t hdd_add_sta_info_sap(struct sk_buff *skb, int8_t rssi,
 	if (!nla_attr)
 		goto fail;
 
-	if (nla_put_u8(skb, NL80211_STA_INFO_SIGNAL, rssi)) {
+	if (nla_put_u8(skb, NL80211_STA_INFO_SIGNAL,
+		       rssi - HDD_NOISE_FLOOR_DBM)) {
 		hdd_err("put fail");
 		goto fail;
 	}