Bluetooth: Drop L2CAP chan reference if ERTM ack_timer fired

Reference counter was incremented when starting ack timer but
decremented only when clearing timer, not when timer fired.

Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>