iwlwifi: audit singles frame from AGG queue in RS

The rate scaling won't treat the information in a frame
with IEEE80211_TX_CTL_AMPDU set if IEEE80211_TX_STAT_AMPDU
is cleared. But all the frames coming from an AGG tx queue
have IEEE80211_TX_CTL_AMPDU set, and IEEE80211_TX_STAT_AMPDU
is set only if the frame was sent in an AMPDU.
This means that all the data in frames in AGG tx queues that
aren't sent as an AMPDU is thrown away.
This is even more harmful when in bad link conditions, the
frames are sent in an AMPDU and then finally sent as single
frame. So a lot of failures weren't reported and the rate
scaling got stuck in high rates leading to very poor
connectivity.

Fix that by clearing IEEE80211_TX_CTL_AMPDU when the frame
isn't part of an AMPDU.

Change-Id: I769f79d3d2a4ebcb71af78e7e17a2fb3a12f30f7
Cc: stable@vger.kernel.org
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>