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>