device tree: Fix cppcheck warning

Fix this cppcheck warning:

Checking device_tree.c...
device_tree.c:216: style:
 Checking if unsigned variable 'r' is less than zero.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

device_tree.c

@@ -213,7 +213,7 @@ uint32_t qemu_devtree_get_phandle(void *fdt, const char *path)
     uint32_t r;
 
     r = fdt_get_phandle(fdt, findnode_nofail(fdt, path));
-    if (r <= 0) {
+    if (r == 0) {
         fprintf(stderr, "%s: Couldn't get phandle for %s: %s\n", __func__,
                 path, fdt_strerror(r));
         exit(1);