serial: 8250: extract serial8250_set_trigger_for_slow_speed()

serial8250_do_set_termios() consists of many registers and up flags
settings. Extract all these into separate functions. This time, setting
of trigger level for slow speeds.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20250611100319.186924-20-jirislaby@kernel.org


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