Ok, makes sense. If I split this out to the ISA driver, I would
probably also disallow changing io_type, as ISA 8250 ports are
all based on PIO mode.

Alas not, they also come in MMIO form now and then, not often but just
enough to be annoyingly unignorable. However the ISA driver can handle
that 'in-house' and I've never seen anything but PIO or simple MMIO.

Alan