mac80211: clear SSID when stopping AP

When AP interface is stopped ssid_len in the BSS configuration
isn't cleared which can confuse drivers when switching modes.
Set the length to zero when stopping the AP interface.

Signed-off-by: Marek Puzyniak <marek.puzyniak@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

diff --cc net/mac80211/cfg.c
Simple merge