[PARISC] pdc_stable: More robust sysfs error checking

pdc_stable 0.10:
As mentioned on LKML, pdc_stable wasn't checky enough on the return
values of some calls. This patch makes it more robust to errors when
registering objects in sysfs.

Signed-off-by: Thibaut VARENE <varenet@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>