qcacmn: Provide proper age value for Assoc STA Link Metrics

Use the time stamp taken during the updating of RSSI value to
calculate the time elapsed when the IOCTL call happens. Return
that value as delta age in ms.

Change-Id: I96d36a7a132b7a327d9d5a5ba3a318a82889ebd5
CRs-Fixed: 2394910

dp/inc/cdp_txrx_stats_struct.h

@@ -528,6 +528,7 @@ struct cdp_tx_stats {
  * @rx_discard: packets discard in rx
  * @rx_ratecode: Rx rate code of last frame
  * @rx_flags: rx flags
+ * @rx_rssi_measured_time: Time at which rssi is measured
  */
 struct cdp_rx_stats {
 	struct cdp_pkt_info to_stack;
@@ -583,6 +584,7 @@ struct cdp_rx_stats {
 	uint32_t rx_discard;
 	uint32_t rx_ratecode;
 	uint32_t rx_flags;
+	uint32_t rx_rssi_measured_time;
 };
 
 /* struct cdp_tx_ingress_stats - Tx ingress Stats