mtd: core: set some defaults when dev.parent is set

If a parent device is set, add_mtd_device() has enough knowledge to fill
in some sane default values for the module name and owner. Do so if they
aren't already set.

Signed-off-by: Frans Klaver <fransklaver@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>