cfg80211: re-enable CSA for drivers that support it

This reverts commit dda444d52496aa8ddc501561bca580f1374a96a9.

Channel switching code has been reworked and
improved significantly since the time original
locking issues were found.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/core.c

@@ -492,12 +492,6 @@ int wiphy_register(struct wiphy *wiphy)
 	int i;
 	u16 ifmodes = wiphy->interface_modes;
 
-	/*
-	 * There are major locking problems in nl80211/mac80211 for CSA,
-	 * disable for all drivers until this has been reworked.
-	 */
-	wiphy->flags &= ~WIPHY_FLAG_HAS_CHANNEL_SWITCH;
-
 #ifdef CONFIG_PM
 	if (WARN_ON(wiphy->wowlan &&
 		    (wiphy->wowlan->flags & WIPHY_WOWLAN_GTK_REKEY_FAILURE) &&