rt2x00: Fix invalid DMA free

Be more strict when using the queue_entry_priv_pci_rx
and queue_entry_priv_pci_tx structures. Only use a
particular type that matches the queue type.

When freeing the DMA the priv_tx->data and priv_tx->dma
was used. This is incorrect since the start of the DMA
was in fact the priv_tx->desc pointer. Instead of
recalculating the dma_addr_t for the DMA start this
patch will swap the data and descriptor part of the
allocated memory.

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