mac80211: release channel on auth failure

There were a few rare cases when upon
authentication failure channel wasn't released.
This could cause stale pointers to remain in
chanctx assigned_vifs after interface removal and
trigger general protection fault later.

This could be triggered, e.g. on ath10k with the
following steps:

 1. start an AP
 2. create 2 extra vifs on ath10k host
 3. connect vif1 to the AP
 4. connect vif2 to the AP
    (auth fails because ath10k firmware isn't able
     to maintain 2 peers with colliding AP mac
     addresses across vifs and consequently
     refuses sta_info_insert() in
     ieee80211_prep_connection())
 5. remove the 2 extra vifs
 6. goto step 2; at step 3 kernel was crashing:

 general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC
 Modules linked in: ath10k_pci ath10k_core ath
 ...
 Call Trace:
  [<ffffffff81a2dabb>] ieee80211_check_combinations+0x22b/0x290
  [<ffffffff819fb825>] ? ieee80211_check_concurrent_iface+0x125/0x220
  [<ffffffff8180f664>] ? netpoll_poll_disable+0x84/0x100
  [<ffffffff819fb833>] ieee80211_check_concurrent_iface+0x133/0x220
  [<ffffffff81a0029e>] ieee80211_open+0x3e/0x80
  [<ffffffff817f2d26>] __dev_open+0xb6/0x130
  [<ffffffff817f3051>] __dev_change_flags+0xa1/0x170
 ...
 RIP  [<ffffffff81a23140>] ieee80211_chanctx_radar_detect+0xa0/0x170

 (gdb) l * ieee80211_chanctx_radar_detect+0xa0
 0xffffffff81a23140 is in ieee80211_chanctx_radar_detect (/devel/src/linux/net/mac80211/util.c:3182).
 3177             */
 3178            WARN_ON(ctx->replace_state == IEEE80211_CHANCTX_REPLACES_OTHER &&
 3179                    !list_empty(&ctx->assigned_vifs));
 3180
 3181            list_for_each_entry(sdata, &ctx->assigned_vifs, assigned_chanctx_list)
 3182                    if (sdata->radar_required)
 3183                            radar_detect |= BIT(sdata->vif.bss_conf.chandef.width);
 3184
 3185            return radar_detect;

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