ath9k: Fix usage of driver-private space in tx_info

The ieee80211_tx_info_clear_status() helper also clears the rate counts and
the driver-private part of struct ieee80211_tx_info, so using it breaks
quite a few other things. So back out of using it, and instead define a
ath-internal helper that only clears the area between the
status_driver_data and the rates info. Combined with moving the
ath_frame_info struct to status_driver_data, this avoids clearing anything
we shouldn't be, and so we can keep the existing code for handling the rate
information.

While fixing this I also noticed that the setting of
tx_info->status.rates[tx_rateindex].count on hardware underrun errors was
always immediately overridden by the normal setting of the same fields, so
rearrange the code so that the underrun detection actually takes effect.

The new helper could be generalised to a 'memset_between()' helper, but
leave it as a driver-internal helper for now since this needs to go to
stable.

Cc: stable@vger.kernel.org
Reported-by: Peter Seiderer <ps.report@gmx.net>
Fixes: 037250f0 ("ath9k: Properly clear TX status area before reporting to mac80211")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Tested-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220404204800.2681133-1-toke@toke.dk