mac80211: fix a memory leak on sta rate selection table

commit 53d045258ee2e38b1e882617cb0799a04d05f5fa upstream.

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: 0d528d85 ("mac80211: improve the rate control API")
[add commit log entry, remove pointless NULL check]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>