ath10k: htt_rx: fix signedness bug in ath10k_update_per_peer_tx_stats

Currently, the error handling for the call to function
ath10k_get_legacy_rate_idx() doesn't work because
*rate_idx* is of type u8 (8 bits, unsigned), which
makes it impossible for it to hold a value less
than 0.

Fix this by changing the type of variable *rate_idx*
to s8 (8 bits, signed).

Addresses-Coverity-ID: 1473914 ("Unsigned compared against 0")
Fixes: 0189dbd7 ("ath10k: get the legacy rate index to update the txrate table")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

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

@@ -2753,7 +2753,8 @@ ath10k_update_per_peer_tx_stats(struct ath10k *ar,
 				struct ath10k_per_peer_tx_stats *peer_stats)
 {
 	struct ath10k_sta *arsta = (struct ath10k_sta *)sta->drv_priv;
-	u8 rate = 0, rate_idx = 0, sgi;
+	u8 rate = 0, sgi;
+	s8 rate_idx = 0;
 	struct rate_info txrate;
 
 	lockdep_assert_held(&ar->data_lock);