rt2x00: Update comment about the AMPDU flag in the TXWI

During testing with AMPDUs it turned out that the rt2800 hw will aggregate
consecutive frames with the same RA and TID when the first frame in a
possible aggregate has set AMPDU=1 in the TXWI. If a following frame has
set AMPDU=0 in its TXWI it might sill end up in the aggregate of the
previous frame. Update the comment accordingly.

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>