mac80211: fix direct probe loop on ieee80211_work_purge

If authentication has already been performed when the WLAN interface is
stopped, (sometimes) the ieee80211_work_purge would corrupt some
ieee80211_work-structures. The outcome is this (cleaned up):

[ 2252.398681] WARNING: at net/mac80211/work.c:995 ieee80211_work_purge
[ 2252.466430] Backtrace:
[ 2252.529266] (ieee80211_work_purge+0x0/0xcc [mac80211])
[ 2252.546875] (ieee80211_stop+0x0/0x4c0 [mac80211])

Additionally, one would get this, going on regarless of the WLAN interface
state, going on forever:

[ 2252.859985] wlan0: direct probe to 00:90:4c:60:04:00 (try -996717525)
[ 2253.055419] wlan0: direct probe to 00:90:4c:60:04:00 (try -996717524)
[ 2253.250610] wlan0: direct probe to 00:90:4c:60:04:00 (try -996717523)
[ 2253.446014] wlan0: direct probe to 00:90:4c:60:04:00 (try -996717522)
[ 2253.641357] wlan0: direct probe to 00:90:4c:60:04:00 (try -996717521)

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>