[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: NRalf Baechle <ralf@linux-mips.org>

arch/mips/kernel/setup.c

@@ -331,6 +331,7 @@ static void __init bootmem_init(void)
 	/*
 	 * Determine low and high memory ranges
 	 */
+	max_pfn = max_low_pfn;
 	if (max_low_pfn > PFN_DOWN(HIGHMEM_START)) {
 #ifdef CONFIG_HIGHMEM
 		highstart_pfn = PFN_DOWN(HIGHMEM_START);