mac80211: fix crash on getting sta info with uninitialized rate control

If the expected throughput is queried before rate control has been
initialized, the minstrel op for it will crash while trying to access
the rate table.
Check for WLAN_STA_RATE_CONTROL before attempting to use the rate
control op.

Reported-by: Jean-Pierre Tosoni <jp.tosoni@acksys.fr>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>