mac80211: allow overriding HT STBC capabilities

Allow user to override STBC configuration for Rx and Tx spatial streams.
In practice RX/TX STBC settings can be modified using appropriate
options in wpa_supplicant configuration file:
  tx_stbc=-1..1
  rx_stbc=-1..3

This functionality has been added to wpa_supplicant in commit cdeea70f59d0.

In FullMAC case these STBC options are passed to drivers by cfg80211
connect callback in fields of cfg80211_connect_params structure.
However for mac80211 drivers, e.g. for mac80211_hwsim,
overrides for STBC settings are ignored.

The reason why RX/TX STBC capabilities are not modified for mac80211
drivers is as follows. All drivers need to specify supported HT/VHT
overrides explicitly: see ht_capa_mod_mask and vht_capa_mod_mask fields
of wiphy structure. Only supported overrides will be passed to drivers by
cfg80211_connect and cfg80211_mlme_assoc operations: see bitwise 'AND'
performed by cfg80211_oper_and_ht_capa and cfg80211_oper_and_vht_capa.

This commit adds RX/TX STBC HT capabilities to mac80211_ht_capa_mod_mask,
allowing their modifications, as well as applies requested STBC
modifications in function ieee80211_apply_htcap_overrides.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/ht.c

@@ -107,6 +107,14 @@ void ieee80211_apply_htcap_overrides(struct ieee80211_sub_if_data *sdata,
 	__check_htcap_enable(ht_capa, ht_capa_mask, ht_cap,
 			     IEEE80211_HT_CAP_40MHZ_INTOLERANT);
 
+	/* Allow user to enable TX STBC bit  */
+	__check_htcap_enable(ht_capa, ht_capa_mask, ht_cap,
+			     IEEE80211_HT_CAP_TX_STBC);
+
+	/* Allow user to configure RX STBC bits */
+	if (ht_capa_mask->cap_info & IEEE80211_HT_CAP_RX_STBC)
+		ht_cap->cap |= ht_capa->cap_info & IEEE80211_HT_CAP_RX_STBC;
+
 	/* Allow user to decrease AMPDU factor */
 	if (ht_capa_mask->ampdu_params_info &
 	    IEEE80211_HT_AMPDU_PARM_FACTOR) {

net/mac80211/main.c

@@ -478,6 +478,8 @@ static const struct ieee80211_ht_cap mac80211_ht_capa_mod_mask = {
 				IEEE80211_HT_CAP_MAX_AMSDU |
 				IEEE80211_HT_CAP_SGI_20 |
 				IEEE80211_HT_CAP_SGI_40 |
+				IEEE80211_HT_CAP_TX_STBC |
+				IEEE80211_HT_CAP_RX_STBC |
 				IEEE80211_HT_CAP_LDPC_CODING |
 				IEEE80211_HT_CAP_40MHZ_INTOLERANT),
 	.mcs = {