rt2x00: In debugfs frame dumping allow the TX descriptor to be part of the skb.

Preparation for futher cleanups in the area of properly maintaining the skb
data without fiddling with the skb->data pointer.

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