iwlwifi: pcie: clear command data on freeing

When freeing the structures used for command data, clear their
memory as they may have contained key material at some point.
Also clear the duplicated buffer when freeing it to be safe;
currently key material is never put there but that may change.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>