net: cadence: Demote non-compliant kernel-doc headers

Fixes the following W=1 kernel build warning(s):

 drivers/net/ethernet/cadence/macb_pci.c:3: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 drivers/net/ethernet/cadence/macb_ptp.c:3: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Yang Shen <shenyang39@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/cadence/macb_pci.c

 // SPDX-License-Identifier: GPL-2.0-only
-/**
+/*
  * DOC: Cadence GEM PCI wrapper.
  *
  * Copyright (C) 2016 Cadence Design Systems - https://www.cadence.com

drivers/net/ethernet/cadence/macb_ptp.c

 // SPDX-License-Identifier: GPL-2.0-only
-/**
+/*
  * 1588 PTP support for Cadence GEM device.
  *
  * Copyright (C) 2017 Cadence Design Systems - https://www.cadence.com