qcacmn: Check for enqueue timestamp for Tx delay calculation

The Tx delay computed is very large if the enqueue
timestamp is zero.
Compute Tx delay only when the enqueue timestamp is valid.

Change-Id: Ic4032a6f4a01fc661dc1155a4543cd7fc4216970
CRs-Fixed: 3354639

dp/wifi3.0/dp_tx.c

@@ -4594,6 +4594,8 @@ void dp_tx_compute_delay(struct dp_vdev *vdev, struct dp_tx_desc_s *tx_desc,
 	fwhw_transmit_delay = (uint32_t)(current_timestamp -
 					 timestamp_hw_enqueue);
 
+	if (!timestamp_hw_enqueue)
+		return;
 	/*
 	 * Delay between packet enqueued to HW and Tx completion in ms
 	 */