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