Fix PCI bus detach to delete child devices.

Arguably, bus_generic_detach() should be calling device_delete_children()
as most (if not all) buses should delete child devices when detaching,
but that's a larger problem to solve.

If I had my druthers I'd probably rename bus_generic_detach() to
device_detach_children() and change bus_generic_detach() to call
device_detach_children() followed by device_delete_children().

• the hotplug patches detach (and delete) the PCI bus on card insertion and exercised this change