bootm: fix 'memory-fixup' for vxWorks boot

The check for having a memory node within the fdt blob is made wrong, we
fix this here.

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>

arch/arm/lib/bootm.c

@@ -437,7 +437,7 @@ void boot_prep_vxworks(bootm_headers_t *images)
 
 	if (images->ft_addr) {
 		off = fdt_path_offset(images->ft_addr, "/memory");
-		if (off < 0) {
+		if (off > 0) {
 			if (arch_fixup_fdt(images->ft_addr))
 				puts("## WARNING: fixup memory failed!\n");
 		}