libnvdimm: provide module_nd_driver wrapper

Provide a module_nd_driver() wrapper over simple nd_driver_register()
nd_driver_unregister() combinations in module_init() and module_exit()
respectively.

Note an explicit nd_driver_unregister() had to be implemented as nd
bus drivers did call device_unregister() direcly in the module_exit()
function.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>