iwlwifi: mvm: fix accessing fw_id_to_mac_id

iwlwifi: mvm: fix accessing fw_id_to_mac_id

BugLink: http://bugs.launchpad.net/bugs/1691369
commit a95600294157ca7527ee7c70249fb53e09d8c566 upstream.

Access should be by rcu_dereference. Issue was found by sparse.

Fixes: 65e254821cee ("iwlwifi: mvm: use firmware station PM notification for AP_LINK_PS")
Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>