tty: vt, finish looping on duplicate

When the console is already registered, stop crawling the
registered_con_driver array and return an error immediatelly.

This makes the code more obvious. And we do not need to initialize
retval anymore.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>