mtrr: Don't show an invalid CPU number

When U-Boot did not do the MP init, we don't get an actual CPU number
here. Skip printing it in that case.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

cmd/x86/mtrr.c

@@ -148,7 +148,8 @@ static int do_mtrr(struct cmd_tbl *cmdtp, int flag, int argc,
 			printf("CPU %d:\n", i);
 			ret = do_mtrr_list(reg_count, i);
 			if (ret) {
-				printf("Failed to read CPU %d (err=%d)\n", i,
+				printf("Failed to read CPU %s (err=%d)\n",
+				       i < MP_SELECT_ALL ? simple_itoa(i) : "",
 				       ret);
 				return CMD_RET_FAILURE;
 			}