iwlwifi: set "echo" host command length

"echo" host command has no data, set the length to 0
Signed-off-by: NWey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-core.c

@@ -1434,6 +1434,7 @@ int iwl_cmd_echo_test(struct iwl_priv *priv)
 	int ret;
 	struct iwl_host_cmd cmd = {
 		.id = REPLY_ECHO,
+		.len = { 0 },
 		.flags = CMD_SYNC,
 	};