[Bluetooth]: Call tty_hangup() when DCD is de-asserted

The RFCOMM layer does not handle properly the de-assertation
of CD signal. It should call tty_hangup() to work properly.

Signed-off-by: Timo Teräs <ext-timo.teras@nokia.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: David S. Miller <davem@davemloft.net>