rt2x00: Consistently name skb frame descriptor skbdesc.

The skb frame descriptor is called everywhere skbdesc, except in one
place in rt2x00debug_dump_frame. Change that occurence to have
consistent naming.

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>