fix(st-uart): skip console flush if UART is disabled

Check the USART_CR1_UE bit and if it is 0, the UART is not enabled,
or not clocked (but the read won't freeze the bus and will return 0).
In this case skip the console flush.
If flush didn't happen in time, just exit the function instead of panic.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I5d1ef7e51612b4795e314b2f2da04a514b6c96a0
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/315467
ACI: CITOOLS <MDG-smet-aci-reviews@list.st.com>

Author: Yann-lms

drivers/st/uart/aarch32/stm32_console.S

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2022, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2018-2023, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -217,14 +217,19 @@ func console_stm32_core_flush
 	cmp	r0, #0
 	ASM_ASSERT(ne)
 #endif /* ENABLE_ASSERTIONS */
+	/* Skip flush if UART is not enabled */
+	ldr	r1, [r0, #USART_CR1]
+	tst	r1, #USART_CR1_UE
+	beq	1f
 	/* Check Transmit Data Register Empty */
 	mov	r2, #USART_TIMEOUT
 txe_loop_3:
 	subs	r2, r2, #1
-	beq	plat_panic_handler
+	beq	1f
 	ldr	r1, [r0, #USART_ISR]
 	tst	r1, #USART_ISR_TXE
 	beq	txe_loop_3
+1:
 	bx	lr
 endfunc console_stm32_core_flush