fix(st): flush UART at the end of uart_read()

Add a flush to ensure that the programmer get time to read the last
command sent.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ic1f718d2754f27945f12c04563663b46274810a7
diff --git a/plat/st/common/stm32cubeprogrammer_uart.c b/plat/st/common/stm32cubeprogrammer_uart.c
index d004dcf..e4a5338 100644
--- a/plat/st/common/stm32cubeprogrammer_uart.c
+++ b/plat/st/common/stm32cubeprogrammer_uart.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021-2022, STMicroelectronics - All Rights Reserved
+ * Copyright (c) 2021-2023, STMicroelectronics - All Rights Reserved
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -481,6 +481,8 @@
 		}
 	}
 
+	stm32_uart_flush(&handle.uart);
+
 	return 0;
 }