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>
diff --git a/drivers/net/wireless/rt2x00/rt2800pci.c b/drivers/net/wireless/rt2x00/rt2800pci.c
index f08b6a3..df2c3fb 100644
--- a/drivers/net/wireless/rt2x00/rt2800pci.c
+++ b/drivers/net/wireless/rt2x00/rt2800pci.c
@@ -643,7 +643,7 @@
 	rt2x00_set_field32(&word, TXWI_W0_AMPDU,
 			   test_bit(ENTRY_TXD_HT_AMPDU, &txdesc->flags));
 	rt2x00_set_field32(&word, TXWI_W0_MPDU_DENSITY, txdesc->mpdu_density);
-	rt2x00_set_field32(&word, TXWI_W0_TX_OP, txdesc->ifs);
+	rt2x00_set_field32(&word, TXWI_W0_TX_OP, txdesc->txop);
 	rt2x00_set_field32(&word, TXWI_W0_MCS, txdesc->mcs);
 	rt2x00_set_field32(&word, TXWI_W0_BW,
 			   test_bit(ENTRY_TXD_HT_BW_40, &txdesc->flags));