tty: serial: clean up stop-tx part in altera_uart_tx_chars()

tty: serial: clean up stop-tx part in altera_uart_tx_chars()

The "stop TX" path in altera_uart_tx_chars() is open-coded, so:
* use uart_circ_empty() to check if the buffer is empty, and
* when true, call altera_uart_stop_tx().

Cc: Tobias Klauser <tklauser@distanz.ch>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Acked-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220920052049.20507-3-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>