tty: serial: altera: use altera_jtaguart_stop_tx()

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: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220411104506.8990-4-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>