ath10k: fix potential issue of peer stats allocation

STA number was not restored if OOM happened.

Tested: QCA9984 with firmware ver 10.4-3.9.0.1-00018
Signed-off-by: Zhi Chen <zhichen@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/ath10k/mac.c

@@ -6604,6 +6604,7 @@ static int ath10k_sta_state(struct ieee80211_hw *hw,
 			arsta->tx_stats = kzalloc(sizeof(*arsta->tx_stats),
 						  GFP_KERNEL);
 			if (!arsta->tx_stats) {
+				ath10k_mac_dec_num_stations(arvif, sta);
 				ret = -ENOMEM;
 				goto exit;
 			}