fix(st-uart): correct UART enable test in flush function

Correct test if UART is enabled.
Avoid panic when flush has not happened in the given time. The function
just exits in that case.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I550211d348687b52dcf23b5f13f4afc891801e6a
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/315186
ACI: CITOOLS <MDG-smet-aci-reviews@list.st.com>
ACI: CIBUILD <MDG-smet-aci-builds@list.st.com>

Author: Yann-lms

drivers/st/uart/aarch32/stm32_console.S

@@ -240,13 +240,13 @@ func console_stm32_core_flush
 #endif /* ENABLE_ASSERTIONS */
 	/* Skip flush if UART is not enabled */
 	ldr	r1, [r0, #USART_CR1]
-	ands	r1, r1, #USART_CR1_UE
+	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