acpi: Fix stalled value returned by acpi_get_device() after device deletion

Newbus device reference attached to ACPI handle is not cleared when newbus
device is deleted with devctl(8) delete command. Fix that with calling of
AcpiDetachData() from "child_deleted" bus method like acpi_pci driver does.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D22902