In the past, carl9170 has been plagued by mysterious
ghosts.

e.g.:
 wlan4: deauthenticated from 02:04:d8:3c:ac:c1 (Reason: 0)

Apparently, the AP sent us a bogus deauthentication
notification. But upon closer inspection the
"management frame" turned out to be a corrupted
scrap of an unsuccessful A-MPDU.
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>