MIPS: Remove a warning when PHYS_OFFSET is 0x0

Rewrite the comparison in `else if` statement, case where `min_low_pfn >
ARCH_PFN_OFFSET` has already been checked in the first `if` statement:

  if (min_low_pfn > ARCH_PFN_OFFSET) {

Fix non-fatal warning during compilation using W=1:

arch/mips/kernel/setup.c: In function ‘bootmem_init’:
arch/mips/kernel/setup.c:461:25: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
  } else if (min_low_pfn < ARCH_PFN_OFFSET) {
                         ^

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18176/


Signed-off-by: James Hogan <jhogan@kernel.org>