rt2x00: Cleanup struct skb_frame_desc.

The data and data_len fields aren't really necessary in struct
skb_frame_desc, as they can be deduced from the skb itself.

Signed-off-by: Gertjan van Wingerde <gwingerde@kpnplanet.nl>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>