rt2x00: don't write past the end when writing short descriptors on rt61

The space allocated in the skb for a descriptor is only 24 bytes when
setting up beacons in rt61 so make sure we don't write to the descriptor
words beyond that and corrupt the beacon packet.

Signed-off-by: Adam Baker <linux@baker-net.org.uk>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>