serial: 8250: extract serial8250_startup_special()

serial: 8250: extract serial8250_startup_special()

Let the serial8250_do_startup() code handle the special ports (16C950,
DA830, RSA) startup in a separate function:
serial8250_startup_special().

And instead of multiple if-else-if, use switch-case. So that it can be
easily checked for PORT_RSA now too.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20250611100319.186924-14-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>