mac80211: use rate index in TX control

This patch modifies struct ieee80211_tx_control to give band
info and the rate index (instead of rate pointers) to drivers.
This mostly serves to reduce the TX control structure size to
make it fit into skb->cb so that the fragmentation code can
put it there and we can think about passing it to drivers that
way in the future.

The rt2x00 driver update was done by Ivo, thanks.

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