mac80211: fix a crash in legacy minstrel when no STA exists

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

Backport of r37342

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@37343 3c298f89-4303-0410-b956-a3cf2f4a3e73