serial: 8250: extract serial8250_initialize()

serial8250_do_startup() initializes the ports in the middle of the
function. This code can be separated to serial8250_initialize(), so that
serial8250_do_startup() can be readable again.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250611100319.186924-17-jirislaby@kernel.org


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