mac80211: fix channel switch for chanctx-based drivers

The new_ctx pointer is set only for non-chanctx drivers.  This yielded a
crash for chanctx-based drivers during channel switch finalization:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
  IP: ieee80211_vif_use_reserved_switch+0x71c/0xb00 [mac80211]

Use an adequate chanctx pointer to fix this.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

net/mac80211/chan.c

@@ -1444,7 +1444,7 @@ ieee80211_vif_use_reserved_switch(struct ieee80211_local *local)
 
 			list_del(&sdata->reserved_chanctx_list);
 			list_move(&sdata->assigned_chanctx_list,
-				  &new_ctx->assigned_vifs);
+				  &ctx->assigned_vifs);
 			sdata->reserved_chanctx = NULL;
 
 			ieee80211_vif_chanctx_reservation_complete(sdata);