[MIPS] Initialize max_pfn again.

This was dropped by

commit a0d9e2d891e4cf54676c430da63bd4a17d1cdb80 (lmo)
commit b6f1f0de

 (ko)
Author: Franck Bui-Huu <vagabon.xyz@gmail.com>
Date:   Fri Aug 11 17:51:48 2006 +0200

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>