ath11k: fix uninitialized rate_idx in ath11k_dp_tx_update_txcompl()

The rate_idx which passed to ath11k_debugfs_sta_add_tx_stats() by
ath11k_dp_tx_update_txcompl() is not initialized, add initialization
for it.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-03003-QCAHSPSWPL_V1_V2_SILICONZ_LITE-2

Fixes: 1b8bb94c ("ath11k: report tx bitrate for iw wlan station dump")
Signed-off-by: Wen Gong <quic_wgong@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220209060816.423-1-quic_wgong@quicinc.com

drivers/net/wireless/ath/ath11k/dp_tx.c

@@ -426,7 +426,7 @@ void ath11k_dp_tx_update_txcompl(struct ath11k *ar, struct hal_tx_status *ts)
 	struct ath11k_sta *arsta;
 	struct ieee80211_sta *sta;
 	u16 rate, ru_tones;
-	u8 mcs, rate_idx, ofdma;
+	u8 mcs, rate_idx = 0, ofdma;
 	int ret;
 
 	spin_lock_bh(&ab->base_lock);