mac80211: advertise BF STS according to AP support

Restrict our published beamformee STS capability according
to the AP value.
Some AP show bad behaviour in interoperability testing
when our capabilities are better.
Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mlme.c

@@ -508,6 +508,7 @@ static void ieee80211_add_vht_ie(struct ieee80211_sub_if_data *sdata,
 	u8 *pos;
 	u32 cap;
 	struct ieee80211_sta_vht_cap vht_cap;
+	u32 mask, ap_bf_sts, our_bf_sts;
 
 	BUILD_BUG_ON(sizeof(vht_cap) != sizeof(sband->vht_cap));
 
@@ -535,6 +536,16 @@ static void ieee80211_add_vht_ie(struct ieee80211_sub_if_data *sdata,
 			cpu_to_le32(IEEE80211_VHT_CAP_SU_BEAMFORMER_CAPABLE)))
 		cap &= ~IEEE80211_VHT_CAP_SU_BEAMFORMEE_CAPABLE;
 
+	mask = IEEE80211_VHT_CAP_BEAMFORMEE_STS_MASK;
+
+	ap_bf_sts = le32_to_cpu(ap_vht_cap->vht_cap_info) & mask;
+	our_bf_sts = cap & mask;
+
+	if (ap_bf_sts < our_bf_sts) {
+		cap &= ~mask;
+		cap |= ap_bf_sts;
+	}
+
 	/* reserve and fill IE */
 	pos = skb_put(skb, sizeof(struct ieee80211_vht_cap) + 2);
 	ieee80211_ie_build_vht_cap(pos, &vht_cap, cap);