ACPI / memhotplug: fix memory leak when memory device is unbound from acpi_memhotplug

We allocate memory to store acpi_memory_info, so we should free it before
freeing mem_device.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Reviewed-by: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>