staging: mt29f_spinand: show parent device in sysfs

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

While at it, take advantage of the default owner and name values set by
mtdcore.

Signed-off-by: Frans Klaver <fransklaver@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>