kobject: convert parisc/pdc_stable to use kobject_create

Using a kset for this simple directory is an overkill.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Thibaut VARENE <varenet@parisc-linux.org>
Cc: Matthew Wilcox <matthew@wil.cx>
Cc: Grant Grundler <grundler@parisc-linux.org>
Cc: Kyle McMartin <kyle@parisc-linux.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>