[Bluetooth] Hangup TTY before releasing rfcomm_dev

The core problem is that RFCOMM socket layer ioctl can release
rfcomm_dev struct while RFCOMM TTY layer is still actively using
it. Calling tty_vhangup() is needed for a synchronous hangup before
rfcomm_dev is freed.

Addresses the oops at http://bugzilla.kernel.org/show_bug.cgi?id=7509

Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>