usb: gadget: bdc: fix improper SPDX comment style for header file

For C header files Documentation/process/license-rules.rst
mandates C-like comments (opposed to C source files where
C++ style should be used).

Cc: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1610505748-30616-1-git-send-email-chunfeng.yun@mediatek.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/udc/bdc/bdc.h

-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * bdc.h - header for the BRCM BDC USB3.0 device controller
  *

drivers/usb/gadget/udc/bdc/bdc_cmd.h

-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * bdc_cmd.h - header for the BDC debug functions
  *

drivers/usb/gadget/udc/bdc/bdc_dbg.h

-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * bdc_dbg.h - header for the BDC debug functions
  *

drivers/usb/gadget/udc/bdc/bdc_ep.h

-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * bdc_ep.h - header for the BDC debug functions
  *