Bluetooth: Host: Free ACL RX fragments on disconnection

This function call frees the buffer kept by the host for reassembling L2CAP
PDUs into.

Without this call, the current buffer will eventually be
leaked, leading to a non-functional host due to lack of RX buffers.

The effect is worse when host flow control is not enabled, as the RX
buffer pool is shared with events, which means communication with the
controller is essentially dead.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>