rt2x00: Use the correct size when copying the control info in txdone

The sizeof() operator was incorrectly applied to the pointer, not the struct.

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>