staging: wlan-ng: Fix improper SPDX comment style

The SPDX license identifier should have the form
// SPDX-License-Identifier: <SPDX License Expression>
for a .c source file. File hfa384x_usb.c has instead the form
/* SPDX-License-Identifier: <SPDX License Expression> */
which is the form for C header files. Hence this patch corrects it.
Issue identified by checkpatch.

Signed-off-by: Himadri Pandya <himadri18.07@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>