tty: serial: lpc32xx_hs: use serial_lpc32xx_stop_tx() helper

Instead of open-coding what serial_lpc32xx_stop_tx() already does, call
it in __serial_lpc32xx_tx() directly.

Cc: Vladimir Zapolskiy <vz@mleia.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220224111028.20917-6-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>