rt2x00: Centralize allocation of RX skbs.

In preparation of replacing the statically allocated DMA buffers with
dynamically mapped skbs, centralize the allocation of RX skbs to rt2x00queue.c
and let rt2x00pci already use them.

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