mac80211: fix a null pointer deref when querying station info when rate control is not initialized (#17061)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41801 3c298f89-4303-0410-b956-a3cf2f4a3e73