carl9170: fix spurious transmissions in sniffer mode

Several people have complained about an unusual
and undocumented feature of the AR9170 hardware:

In siffer mode, the hardware generates spurious
ACK frames for every received frame... even
broadcasts.

The reason for this malfunction is unknown:
<http://marc.info/?l=linux-wireless&m=134517238506033

>
But there's a workaround: Instead of the special
sniffer mode, the hardware will be put into
station mode and all rx filters are disabled.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Reported-by: Marco Fonseca <marco@tampabay.rr.com>
Reported-by: Janusz Dziedzic <janusz.dziedzic@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>