tty: serial: altera: use altera_jtaguart_stop_tx()

altera_jtaguart_tx_chars() duplicates what altera_jtaguart_stop_tx()
already does. So instead of the duplication, call the helper instead.

Not only it makes the code cleaner, but it also says what the "if"
really does.

Cc: Tobias Klauser <tklauser@distanz.ch>
Acked-by: NTobias Klauser <tklauser@distanz.ch>
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220411104506.8990-4-jslaby@suse.czSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/altera_jtaguart.c

@@ -168,10 +168,8 @@ static void altera_jtaguart_tx_chars(struct altera_jtaguart *pp)
 		}
 	}
 
-	if (pending == 0) {
-		pp->imr &= ~ALTERA_JTAGUART_CONTROL_WE_MSK;
-		writel(pp->imr, port->membase + ALTERA_JTAGUART_CONTROL_REG);
-	}
+	if (pending == 0)
+		altera_jtaguart_stop_tx(port);
 }
 
 static irqreturn_t altera_jtaguart_interrupt(int irq, void *data)