rt2x00: Use an enum instead of u16 for the rate_mode TX descriptor field

This makes the code less error-prone.
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
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/rt2x00queue.h

@@ -323,7 +323,7 @@ struct txentry_desc {
 		} ht;
 	} u;
 
-	u16 rate_mode;
+	enum rate_modulation rate_mode;
 
 	short retry_limit;