dtc: Fix signedness comparisons warnings: pointer diff

With -Wsign-compare, compilers warn about a mismatching signedness
in comparisons in the function get_node_by_path().

Taking the difference between two pointers results in a signed ptrdiff_t
type, which mismatches the unsigned type returned by strlen().
Since "p" has been returned by a call to strchr() with "path" as its
argument, we know for sure that it's bigger than "path", so the
difference must be positive. So a cast to an unsigned type is valid.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20210611171040.25524-7-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

livetree.c

@@ -526,7 +526,7 @@ struct node *get_node_by_path(struct node *tree, const char *path)
 	p = strchr(path, '/');
 
 	for_each_child(tree, child) {
-		if (p && strprefixeq(path, p - path, child->name))
+		if (p && strprefixeq(path, (size_t)(p - path), child->name))
 			return get_node_by_path(child, p+1);
 		else if (!p && streq(path, child->name))
 			return child;