serial: stm32: fix transmit_chars when tx is stopped

Disables the tx irq  when the transmission is ended and updates stop_tx
conditions for code cleanup.

Fixes: 48a6092f ("serial: stm32-usart: Add STM32 USART Driver")
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -420,13 +420,8 @@ static void stm32_transmit_chars(struct uart_port *port)
 		return;
 	}
 
-	if (uart_tx_stopped(port)) {
-		stm32_stop_tx(port);
-		return;
-	}
-
-	if (uart_circ_empty(xmit)) {
-		stm32_stop_tx(port);
+	if (uart_circ_empty(xmit) || uart_tx_stopped(port)) {
+		stm32_clr_bits(port, ofs->cr1, USART_CR1_TXEIE);
 		return;
 	}
 
@@ -439,7 +434,7 @@ static void stm32_transmit_chars(struct uart_port *port)
 		uart_write_wakeup(port);
 
 	if (uart_circ_empty(xmit))
-		stm32_stop_tx(port);
+		stm32_clr_bits(port, ofs->cr1, USART_CR1_TXEIE);
 }
 
 static irqreturn_t stm32_interrupt(int irq, void *ptr)