qcacmn: Fix tx_stbc stat print issue

set str_buf index to zero before parsing stbc stats from tlv

CRs-Fixed: 2354728
Change-Id: I9100a762a1f40e0ef2fc76ad4a9286b4002a9a82

dp/wifi3.0/dp_stats.c

@@ -751,6 +751,7 @@ static inline void dp_print_tx_peer_rate_stats_tlv(uint32_t *tag_buf)
 				" %d:%d,", i, dp_stats_buf->tx_bw[i]);
 	}
 	DP_TRACE_STATS(FATAL, "tx_bw = %s ", str_buf);
+	index = 0;
 	qdf_mem_set(str_buf, DP_MAX_STRING_LEN, 0x0);
 	for (i = 0; i <  HTT_TX_PDEV_STATS_NUM_MCS_COUNTERS; i++) {
 		index += qdf_snprint(&str_buf[index],