fdt: Clarify the fdt pre-relocation warning

Reword this so it is easier to understand.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/core/ofnode.c b/drivers/core/ofnode.c
index 1d4ab5b..dee890b 100644
--- a/drivers/core/ofnode.c
+++ b/drivers/core/ofnode.c
@@ -72,7 +72,7 @@
 		}
 	} 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();
 		}
 	}