ACPI / memhotplug: remove redundant logic of acpi memory hotadd

When memory hotadd, acpi_memory_enable_device has already been done
at drv->ops.add (acpi_memory_device_add), no need to do it again
at notify callback.

At acpi_memory_enable_device, acpi_memory_get_device_resources
is also a redundant action, since it has been done at drv->ops.add.

Signed-off-by: Liu Jinsong <jinsong.liu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>