USB: serial: console: fix reported terminal settings

The USB-serial console implementation has never reported the actual
terminal settings used. Despite storing the corresponding cflags in its
struct console, these were never honoured on later tty open() where the
tty termios would be left initialised to the driver defaults.

Unlike the serial console implementation, the USB-serial code calls
subdriver open() already at console setup. While calling set_termios()
and write() before open() looks like it could work for some USB-serial
drivers, others definitely do not expect this, so modelling this after
serial core is going to be intrusive, if at all possible.

Instead, use a (renamed) tty helper to save the termios data used at
console setup so that the tty termios reflects the actual terminal
settings after a subsequent tty open().

Note that the calls to tty_init_termios() (tty_driver_install()) and
tty_save_termios() are serialised using the disconnect mutex.

This specifically fixes a regression that was triggered by a recent
change adding software flow control to the pl2303 driver: a getty trying
to disable flow control while leaving the baud rate unchanged would now
also set the baud rate to the driver default (prior to the flow-control
change this had been a noop).

Fixes: 7041d9c3

 ("USB: serial: pl2303: add support for tx xon/xoff flow control")
Cc: stable <stable@vger.kernel.org>	# 4.18
Cc: Florian Zumbiehl <florz@florz.de>
Reported-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>