fdt: Clarify the fdt pre-relocation warning

Reword this so it is easier to understand.

Signed-off-by: Simon Glass <sjg@chromium.org>

drivers/core/ofnode.c

@@ -72,7 +72,7 @@ static oftree oftree_ensure(void *fdt)
 		}
 	} else {
 		if (fdt != gd->fdt_blob) {
-			log_debug("Cannot only access control FDT before relocation\n");
+			log_debug("Only the control FDT can be accessed before relocation\n");
 			return oftree_null();
 		}
 	}