iwlwifi: don't WARN when calling iwl_get_shared_mem_conf with RF-Kill

iwl_mvm_send_cmd returns 0 when the command won't be sent
because RF-Kill is asserted. Do the same when we call
iwl_get_shared_mem_conf since it is not sent through
iwl_mvm_send_cmd but directly calls the transport layer.

Cc: stable@vger.kernel.org
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>