iwlwifi: fix monitor mode FCS flag

When the firmware is in SNIFFER mode, it leaves
the FCS at the end of frame. Not telling mac80211
means it won't add the right flag to the radiotap
header and that confuses wireshark.

Since mac80211 doesn't have a per-packet flag, set
the HW flag dynamically. This works as the monitor
vif can only be present in the driver by itself.

This fixes a regression introduced by my

commit 57897726
Author: Johannes Berg <johannes.berg@intel.com>
Date:   Fri May 11 10:53:18 2012 +0200

    iwlwifi: support explicit monitor interface

Cc: stable@vger.kernel.org [3.5+]
Reported-by: MARK PHILLIPS <mark.phillips@virgin.net>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>