Fix a possible overflow case detected by gcc 4.3.2

.../dtc/libfdt/fdt_sw.c: In function 'fdt_end_node':
.../dtc/libfdt/fdt_sw.c:81: error: assuming signed overflow does not occur when assuming that (X + c) < X is always false

Signed-off-by: Emil Medve <Emilian.Medve@Freescale.com>

libfdt/fdt_sw.c

@@ -70,7 +70,7 @@ static int _fdt_sw_check_header(void *fdt)
 			return err; \
 	}
 
-static void *_fdt_grab_space(void *fdt, int len)
+static void *_fdt_grab_space(void *fdt, size_t len)
 {
 	int offset = fdt_size_dt_struct(fdt);
 	int spaceleft;