backlight: pwm_bl: Convert to use SPDX identifier

Reduce size of duplicated comments by switching to use SPDX identifier.

No functional change.

While here, correct MODULE_LICENSE() string to be aligned with license text.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>