rt2x00: Add WCID to HT TX descriptor

When sending an unencrypted frame to a STA the driver might want to pass
a suitable WCID since we don't have a key index to allow tx status
reports to get properly assigned to the correct STA.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>