iwlwifi: mvm: fix accessing fw_id_to_mac_id

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

Fixes: 65e25482

 ("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>