vdc: write 16bit addresses just like KERNAL does

I read somewhere on the internet that the order of writing the high and low
address bytes actually matter. So we do it just like the KERNAL does:
First write the high address, then the low one:

https://web.archive.org/web/20080202025255/singularcrew.hu/c128rom/C128_ROM_commented.html#$CE0C