ath10k: fix incorrect multicast/broadcast rate setting

Invalid rate code is sent to firmware when multicast rate value of 0 is
sent to driver indicating disabled case, causing broken mesh path.
so fix that.

Tested on QCA9984 with firmware 10.4-3.6.1-00827

Sven tested on IPQ4019 with 10.4-3.5.3-00057 and QCA9888 with 10.4-3.5.3-00053
(ath10k-firmware) and 10.4-3.6-00140 (linux-firmware 2018-12-16-211de167).

Fixes: cd93b83a ("ath10k: support for multicast rate control")
Co-developed-by: Zhi Chen <zhichen@codeaurora.org>
Signed-off-by: Zhi Chen <zhichen@codeaurora.org>
Signed-off-by: Pradeep Kumar Chitrapu <pradeepc@codeaurora.org>
Tested-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

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

@@ -5588,8 +5588,8 @@ static void ath10k_bss_info_changed(struct ieee80211_hw *hw,
 	struct cfg80211_chan_def def;
 	u32 vdev_param, pdev_param, slottime, preamble;
 	u16 bitrate, hw_value;
-	u8 rate, basic_rate_idx;
-	int rateidx, ret = 0, hw_rate_code;
+	u8 rate, basic_rate_idx, rateidx;
+	int ret = 0, hw_rate_code, mcast_rate;
 	enum nl80211_band band;
 	const struct ieee80211_supported_band *sband;
 
@@ -5776,7 +5776,11 @@ static void ath10k_bss_info_changed(struct ieee80211_hw *hw,
 	if (changed & BSS_CHANGED_MCAST_RATE &&
 	    !WARN_ON(ath10k_mac_vif_chan(arvif->vif, &def))) {
 		band = def.chan->band;
-		rateidx = vif->bss_conf.mcast_rate[band] - 1;
+		mcast_rate = vif->bss_conf.mcast_rate[band];
+		if (mcast_rate > 0)
+			rateidx = mcast_rate - 1;
+		else
+			rateidx = ffs(vif->bss_conf.basic_rates) - 1;
 
 		if (ar->phy_capability & WHAL_WLAN_11A_CAPABILITY)
 			rateidx += ATH10K_MAC_FIRST_OFDM_RATE_IDX;