rt2x00: Initialize TX control field in data entries

In the TX path, the driver didn't copy the TX control data structure. Thus, it
was invalid in the TX done handler, causing serious trouble and misbehaviour.

Signed-off-by: Mattias Nissler <mattias.nissler@gmx.de>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>