rt2x00: Fix frame dumping for USB devices.

We forgot to clear the SKBDESC_DESC_IN_SKB when the descriptor was removed
from the front of the skb.

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