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.
Incidentally, it seems the owner field in the concatenated mtds is not
actually used, so this shouldn't make much of a difference anyway.
Signed-off-by: Frans Klaver <fransklaver@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>