iwlwifi: change WEP key protection to use mutex

For later station notification support we would like WEP key setting to be
done synchronously always. Currently all places from which WEP key is set
can sleep, but the usage of sta_lock prevents it to do so. Modify the
locking to use priv->mutex instead and thus enable this call to sleep.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>