tty: Add missing break statement

Being miss in first place.

For tty-major we either should understand the driver
we're handling, in the code above it's VT driver, so if minor
is pointing somewhere else -- it's unknown tty.

We need to look into every serial device we're going to
support if we can c/r it without problem. Thus 'break' prevent
us from potential problems.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>