[Bluetooth] Check that device is in rfcomm_dev_list before deleting

If RFCOMM_RELEASE_ONHUP flag is on and rfcomm_release_dev is called
before connection is closed, rfcomm_dev is deleted twice from the
rfcomm_dev_list and refcount is messed up. This patch adds a check
before deleting device that the device actually is listed.

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