tty: update obsolete termios comment

Update an obsolete comment referring to the termios_locked structure
which was removed over a decade ago by commit fe6e29fd

 ("tty:
simplify ktermios allocation").

While at it, fix the "Thus" typo.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>