mac80211: force use_short_slot=true for 5GHz

Normally 5GHz does not have a concept of long vs short slot time,
however the slot time that it ends up using is the same as for 2.4 GHZ
and use_short_slot == true
Because of that, it makes more sense to force use_short_slot = true
whenever 5 GHz is being used, so that this particular check does not
have to be in every single driver that uses this flag.
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/cfg.c

@@ -1085,6 +1085,13 @@ static int ieee80211_change_bss(struct wiphy *wiphy,
 			params->use_short_preamble;
 		changed |= BSS_CHANGED_ERP_PREAMBLE;
 	}
+
+	if (!sdata->vif.bss_conf.use_short_slot &&
+	    sdata->local->hw.conf.channel->band == IEEE80211_BAND_5GHZ) {
+		sdata->vif.bss_conf.use_short_slot = true;
+		changed |= BSS_CHANGED_ERP_SLOT;
+	}
+
 	if (params->use_short_slot_time >= 0) {
 		sdata->vif.bss_conf.use_short_slot =
 			params->use_short_slot_time;

net/mac80211/mlme.c

@@ -670,6 +670,8 @@ static u32 ieee80211_handle_bss_capability(struct ieee80211_sub_if_data *sdata,
 	}
 
 	use_short_slot = !!(capab & WLAN_CAPABILITY_SHORT_SLOT_TIME);
+	if (sdata->local->hw.conf.channel->band == IEEE80211_BAND_5GHZ)
+		use_short_slot = true;
 
 	if (use_protection != bss_conf->use_cts_prot) {
 		bss_conf->use_cts_prot = use_protection;