rt2x00: Remove ieee80211_tx_control argument from write_tx_desc()

Move the last remaining information details read from ieee80211_tx_control
in the drivers to the txentry_desc structure. After this we can
remove ieee80211_tx_control from the argument list for the callback function,
which makes it easier when the control information is moved into skb->cb

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