Bluetooth: Fix disconnecting L2CAP when a credits overflow occurs

The L2CAP specification requires us to disconnect an L2CAP channel if
the remote side gives us credits beyond 65535. This patch makes sure we
disconnect the channel in such a situation.

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