mac80211: fix a memory leak on sta rate selection table

If the rate control algorithm uses a selection table, it
is leaked when the station is destroyed - fix that.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Reported-by: Christophe Prévotaux <cprevotaux@nltinc.com>
Fixes: 0d528d85c519 ("mac80211: improve the rate control API")
Cc: stable@vger.kernel.org # v3.10+
[add commit log entry, remove pointless NULL check]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>