MIPS: Fix flood of warnings about comparsion being always true.

./arch/mips/include/asm/page.h:204:13: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]

The default value of ARCH_PFN_OFFSET is 0 thus triggering this warning
for all platforms using the default value.
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/page.h

@@ -200,8 +200,9 @@ static inline int pfn_valid(unsigned long pfn)
 {
 	/* avoid <linux/mm.h> include hell */
 	extern unsigned long max_mapnr;
+	unsigned long pfn_offset = ARCH_PFN_OFFSET;
 
-	return pfn >= ARCH_PFN_OFFSET && pfn < max_mapnr;
+	return pfn >= pfn_offset && pfn < max_mapnr;
 }
 
 #elif defined(CONFIG_SPARSEMEM)