This routine is called when the system is about to be shut down. It is expected to
bring the hardware to some consistent state. For most of the ISA devices no special
action is required, so the function is not really necessary because the device will be
re-initialized on reboot anyway. But some devices have to be shut down with a special
procedure, to make sure that they will be properly detected after soft reboot (this is
especially true for many devices with proprietary identification protocols). In any case
disabling DMA and interrupts in the device registers and stopping any ongoing transfers
is a good idea. The exact action depends on the hardware, so we do not consider it here
in any detail.
