wifi: mt76: fix available_antennas setting

Check if available_antennas_tx and available_antennas_rx are already set
during the per-chip initialization phase; otherwise, they could be
overwritten with incorrect values.

Fixes: 69d54ce7491d ("wifi: mt76: mt7996: switch to single multi-radio wiphy")
Signed-off-by: Shayne Chen <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Felix Fietkau <[e-mail]>

drivers/net/wireless/mediatek/mt76/mac80211.c

···
 	wiphy_ext_feature_set(wiphy, NL80211_EXT_FEATURE_AIRTIME_FAIRNESS);
 	wiphy_ext_feature_set(wiphy, NL80211_EXT_FEATURE_AQL);
 
-	wiphy->available_antennas_tx = phy->antenna_mask;
-	wiphy->available_antennas_rx = phy->antenna_mask;
+	if (!wiphy->available_antennas_tx)
+		wiphy->available_antennas_tx = phy->antenna_mask;
+	if (!wiphy->available_antennas_rx)
+		wiphy->available_antennas_rx = phy->antenna_mask;
 
 	wiphy->sar_capa = &mt76_sar_capa;
 	phy->frp = devm_kcalloc(dev->dev, wiphy->sar_capa->num_freq_ranges,