serial: stm32: update transmission complete error message in shutdown

The transmission complete error message provides the status of the
ISR_USART_TC bit. This bit, when set, indicates that the transmission
has not been completed.
The bit status indication is not a very understandable information.
The error message sent on console should indicate that the transmission is
not complete, instead of providing USART_TC bit status.
Update the error message and add a comment for better understanding.
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210106162203.28854-9-erwan.leray@foss.st.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -687,8 +687,9 @@ static void stm32_usart_shutdown(struct uart_port *port)
 					 isr, (isr & USART_SR_TC),
 					 10, 100000);
 
+	/* Send the TC error message only when ISR_TC is not set */
 	if (ret)
-		dev_err(port->dev, "transmission complete not set\n");
+		dev_err(port->dev, "Transmission is not complete\n");
 
 	stm32_usart_clr_bits(port, ofs->cr1, val);