rt2x00: rt2800: use correct txop value in tx descriptor

rt2800 devices use a different enumeration to specify what IFS values should
be used on frame transmission compared to the other rt2x00 devices. Hence,
create a new enum called txop that contains the valid values.

Furthermore use the appropriate txop values as found in the ralink drivers:
- TXOP_BACKOFF for management frames
- TXOP_SIFS for subsequent fragments in a burst
- TXOP_HTTXOP for all data frames

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

drivers/net/wireless/rt2x00/rt2x00queue.h

@@ -299,6 +299,7 @@ enum txentry_desc_flags {
  * @retry_limit: Max number of retries.
  * @aifs: AIFS value.
  * @ifs: IFS value.
+ * @txop: IFS value for 11n capable chips.
  * @cw_min: cwmin value.
  * @cw_max: cwmax value.
  * @cipher: Cipher type used for encryption.
@@ -328,6 +329,7 @@ struct txentry_desc {
 	short retry_limit;
 	short aifs;
 	short ifs;
+	short txop;
 	short cw_min;
 	short cw_max;