mtd: maps: rbtx4939-flash: show parent device in sysfs

Fix a bug where mtd parent device symlinks aren't shown in sysfs.

While at it, make use of the default owner value set by mtdcore.

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