serial: 8250: extract serial8250_set_TRG_levels()

serial: 8250: extract serial8250_set_TRG_levels()

serial8250_do_startup() contains peculiar trigger levels setup for
special ports (16850, ALTR_16550_*). Move this away to a separate
function: serial8250_set_TRG_levels().

And use switch-case instead of 'if's.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20250611100319.186924-15-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>