rt2x00: Optimize TX descriptor memory layout

Some fields only need to be u8 and for ifs and txop we can use the
already available enums.

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>