serial: 8250: kill __do_stop_tx()

There seems to be little reason for __do_stop_tx() to exits on its own.
It is rather simple and is only called from __stop_tx(). Thus, move its
logic into __stop_tx().

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220607084154.8172-1-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>