driver core: register_memory/unregister_memory clean ups and bugfix

register_memory()/unregister_memory() never gets called with
"root". unregister_memory() is accessing kobject_name of
the object just freed up. Since no one uses the code,
lets take the code out. And also, make register_memory() static.

Another bug fix - before calling unregister_memory()
remove_memory_block() gets a ref on kobject. unregister_memory()
need to drop that ref before calling sysdev_unregister().

Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>