mac80211: notify drivers on sta rate table changes

This allows drivers with a firmware or chip-based rate lookup table to
use the most recent default rate selection without having to get it from
per-packet data or explicit ieee80211_get_tx_rate calls

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>