Bluetooth: Fix disconnecting L2CAP channel for credits violation

The L2CAP specification requires us to disconnect a channel if the
remote device sends us data when it doesn't have any credits to do so.
This patch makes sure that we send the appropriate L2CAP Disconnect
request in this situation.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>