gpiobus: use bus_generic_detach when detaching

[vexeduxr]

Summary

Use bus_generic_detach instead of bus_detach_children so the children
actually get deleted.

[vexeduxr]

This fixes the BUS_DELETED_CHILD call for acpi_gpiobus. I think it might be papering over a bug though.
When device_delete_child is called, say with device_delete_child(gpio0, gpiobus0), it calls device_detach on gpiobus0. It then goes through it's children and recursively calls device_delete_child on them. When one of these children reach the BUS_CHILD_DELETED call, gpiobus0 has already been detached, so the call never reaches it's implementation.