[Bluetooth] Keep rfcomm_dev on the list until it is freed

This patch changes the RFCOMM TTY release process so that the TTY is kept
on the list until it is really freed. A new device flag is used to keep
track of released TTYs.

Signed-off-by: Ville Tervo <ville.tervo@nokia.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>