mac80211: driver can remain on channel if not using chan_ctx

Some of the drivers are not using channel context, but let the
stack to control/switch channels instead. For such cases, driver
can still remain on channel because the mac80211 stack actually
supports it.

The stack will check if the driver is using chan_ctx and has
ops->remain_on_channel been hooked. Otherwise it will start its
ROC work to remain on channel. So, even if the driver is not
using chan_ctx, the driver is still capable of doing remain on
channel.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Link: https://lore.kernel.org/r/20200312074337.16198-1-yhchuang@realtek.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/main.c

@@ -576,7 +576,7 @@ struct ieee80211_hw *ieee80211_alloc_hw_nm(size_t priv_data_len,
 			WIPHY_FLAG_REPORTS_OBSS |
 			WIPHY_FLAG_OFFCHAN_TX;
 
-	if (ops->remain_on_channel)
+	if (!use_chanctx || ops->remain_on_channel)
 		wiphy->flags |= WIPHY_FLAG_HAS_REMAIN_ON_CHANNEL;
 
 	wiphy->features |= NL80211_FEATURE_SK_TX_STATUS |