fix(drivers/st/clk): use correct return value

The function stm32mp1_clk_init() returns an int. Return a negative
error value if the device tree is not found.

Change-Id: I422d5fea46c4d63d55a5b62e1db154c1f53f41b7
Signed-off-by: Yann Gautier <yann.gautier@st.com>

drivers/st/clk/stm32mp1_clk.c

 /*
- * Copyright (C) 2018-2020, STMicroelectronics - All Rights Reserved
+ * Copyright (C) 2018-2021, STMicroelectronics - All Rights Reserved
  *
  * SPDX-License-Identifier: GPL-2.0+ OR BSD-3-Clause
  */
@@ -1737,7 +1737,7 @@ int stm32mp1_clk_init(void)
 	void *fdt;
 
 	if (fdt_get_address(&fdt) == 0) {
-		return false;
+		return -FDT_ERR_NOTFOUND;
 	}
 
 	/* Check status field to disable security */