rt2x00: Don't use pskb_expand_head()

rt2x00pci allocates DMA for descriptor and data,
rt61pci doesn't use this for the beacon, but it can
use the descriptor part as temporary buffer instead
of using pskb_expand_head().
Using this temporary buffer is obviously much better
then reallocating the skb buffer...

At the same time we can set the data length for the
beacon queue at 0, to make sure no DMA is allocated for
data (but just for the descriptor).

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