powerpc/8xx: remove remaining unnecessary code in FixupDAR

Since commit 33fb845a ("powerpc/8xx: Don't use MD_TWC for walk"), MD_EPN and
MD_TWC are not writen anymore in FixupDAR so saving r3 has become useless.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <scottwood@freescale.com>