rt2x00: Work around hw aggregation oddity in rt2800

If a frame is not meant to be sent as AMPDU or part of it the hw might
still decide to aggregate this frame if a previous frame started an
AMPDU. However, this will limit the usefulness of the reported tx rate
since the reported rate will be the one specified in the TXWI of the
first frame and thus it is not possible to reliably caculate the
number of retrys by substracting the reported tx rate from the tx rate
in the TXWI.

To fix this issue, only report the successful rate for frames that were
not meant to be aggregated but ended up in an aggregate.

Example:
Frame A (MCS7, AMPDU=1) B (MCS7, AMPDU=1) C (MCS12, AMDPU=0, PROBE_RATE)

Although frame C shoudn't be aggregated the hw might sill put it
into an AMPDU together with A and B. If the transmission succeeds the tx
status will contain MCS7 for all three frames. In that case we should
only report MCS7 as success rate and avoid reporting MCS12-MCS8 as
failed tx attempts as this will affect the future rate control
decisions.

This oddity might strike us in other scenarious as well but the most
common "wrong" report happened for frames used to probe a different tx
rate.

This improves the rate control decisions notable.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>