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>

drivers/net/wireless/rt2x00/rt2800.h

@@ -2003,7 +2003,13 @@ struct mac_iveiv_entry {
  *     duplicate the frame to both channels).
  * STBC: 1: STBC support MCS =0-7, 2,3 : RESERVED
  * AMPDU: 1: this frame is eligible for AMPDU aggregation, the hw will
- *        aggregate consecutive frames with the same RA and QoS TID.
+ *        aggregate consecutive frames with the same RA and QoS TID. If
+ *        a frame A with the same RA and QoS TID but AMPDU=0 is queued
+ *        directly after a frame B with AMPDU=1, frame A might still
+ *        get aggregated into the AMPDU started by frame B. So, setting
+ *        AMPDU to 0 does _not_ necessarily mean the frame is sent as
+ *        MPDU, it can still end up in an AMPDU if the previous frame
+ *        was tagged as AMPDU.
  */
 #define TXWI_W0_FRAG			FIELD32(0x00000001)
 #define TXWI_W0_MIMO_PS			FIELD32(0x00000002)