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 <[e-mail]>
Reviewed-by: James Hogan <[e-mail]>
Cc: Ralf Baechle <[e-mail]>
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/18176/
Signed-off-by: James Hogan <[e-mail]>

arch/mips/kernel/setup.c

···
 		pr_info("Wasting %lu bytes for tracking %lu unused pages\n",
 			(min_low_pfn - ARCH_PFN_OFFSET) * sizeof(struct page),
 			min_low_pfn - ARCH_PFN_OFFSET);
-	} else if (min_low_pfn < ARCH_PFN_OFFSET) {
+	} else if (ARCH_PFN_OFFSET - min_low_pfn > 0UL) {
 		pr_info("%lu free pages won't be used\n",
 			ARCH_PFN_OFFSET - min_low_pfn);
 	}