rt2x00: rt2x00queue: add priv_size field to struct data_queue

Add a new field into struct data_queue and store
the size of the per-queue_entry private data in
that. Additionally, use the new field in the
rt2x00queue_alloc_entries function to compute
the size of the queue entries for a given queue.

The patch does not change the current behaviour
but makes it possible to remove the queue_desc
parameter of the rt2x00queue_alloc_entries function.
That will be done by a subsequent patch.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>