USB: serial: fix initial-termios handling

The USB-serial driver init_termios callback is used to override the
default initial terminal settings provided by USB-serial core.

After a bug was fixed in the original implementation introduced by
commit fe1ae7fd ("tty: USB serial termios bits"), the init_termios
callback was no longer called just once on first use as intended but
rather on every (first) open.

This specifically meant that the terminal settings saved on (final)
close were ignored when reopening a port for drivers overriding the
initial settings.

Also update the outdated function header referring to the creation of
termios objects.

Fixes: 7e29bb4b ("usb-serial: fix termios initialization logic")
Signed-off-by: Johan Hovold <johan@kernel.org>