rt2x00: Fix rt2800usb TX descriptor writing.

The recent changes to skb handling introduced a bug in the rt2800usb
TX descriptor writing whereby the length of the USB packet wasn't
calculated correctly.
Found via code inspection, as the devices themselves didn't seem to mind.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>