mac80211: notify channel switch at the end of ieee80211_chswitch_post_beacon()

The call to cfg80211_ch_switch_notify() should be at the end of the
ieee80211_chswitch_post_beacon() function, because it should only be
sent if everything succeeded.

Fixes: d04b5ac9 ("cfg80211/mac80211: allow any interface to send channel switch notifications")
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>