serial: 8250: extract serial8250_set_fcr()

serial: 8250: extract serial8250_set_fcr()

serial8250_do_set_termios() consists of many registers and up flags
settings. Extract all these into separate functions. This time, setting
of FCR.

serial8250_do_set_termios() looks sane at this point.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20250611100319.186924-25-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>