iwlwifi: fix locking unbalance in 4965 rate scale

This patch fixes locking unbalance after adding rcu locking
for sta

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>