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

This patch corrects the SPDX License Identifier style in
header files related to USB Serial device configuration.
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>
Signed-off-by: Johan Hovold <johan@kernel.org>