rt2x00: rt2x00queue: setup queue->threshold from queue->limit

Use the queue->limit value instead of the
qdesc->entry_num to compute the threshold.
The two source values are the same and the
data queue descriptor structure will be
removed by a later patch.

Also separate the computation from the rest
of the init code to make further changes
easier.

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>