mac80211_hwsim: Make sure beacon_timer gets deleted

It was possible to trigger a kernel panic because beacon_timer may not
have been deleted in all cases when the kernel module was removed while
hostapd was still running.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>