rt2x00: fix HT TX descriptor settings regression

Since:

commit 36323f81
Author: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Date:   Mon Jul 23 21:33:42 2012 +0200

    mac80211: move TX station pointer and restructure TX

we do not pass sta pointer to rt2x00queue_create_tx_descriptor_ht(),
hence we do not correctly set station WCID and AMPDU density parameters.

Cc: stable@vger.kernel.org # 3.7+
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>