mac80211: do not call rate control .tx_status before .rate_init

Most rate control implementations assume .get_rate and .tx_status are only
called once the per-station data has been fully initialized.
minstrel_ht crashes if this assumption is violated.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Tested-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>