i2c: stm32: Use the correct style for SPDX License Identifier

This patch corrects the SPDX License Identifier style
in header file related to STM32 Driver for I2C hardware
bus support.
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: Wolfram Sang <wsa@the-dreams.de>