powerpc/8xx: remove special handling of CPU6 errata in set_dec()

CPU6 ERRATA is now handled directly in mtspr(), so we can use the
standard set_dec() fonction in all cases.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <oss@buserror.net>