USB: atm: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style in
header file related to USB DSL modem support drivers.
For C header files Documentation/process/license-rules.rst
mandates C-like comments (opposed to C source files where
C++ style should be used).

Changes made by using a script provided by Joe Perches here:
https://lkml.org/lkml/2019/2/7/46.
Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Link: https://lore.kernel.org/r/20200315102844.GA3460@nishadSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/atm/usbatm.h

-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /******************************************************************************
  *  usbatm.h - Generic USB xDSL driver core
  *