scripts: zspdx: fix writing custom license IDs
scripts: zspdx: fix writing custom license IDs

The builtin list function `.sort()` sorts the list in-place and returns
None. As this is an invalid type for iteration, use the builtin `sorted`
function, which returns a sorted copy of the list, which we can iterate
over.

Signed-off-by: Jordan Yates <d74a90e280bbad1cc3646781a4e234b467b60fe1@data61.csiro.au>

zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr