samples: Bluetooth: hci_ipc: __ASSERT on fail to deliver complete message

The hci_ipc sample does not support fragmentation of IPC-messages.
Give an ASSERT if only parts of an IPC-message is delivered.
This would happen for instance when the underlying transport mechanism
cannot hold the complete message.

Background:
The open-amp backend uses RPMSG_BUFFER_SIZE=512

Most applications and controllers use HCI-packets which are at most
255 bytes. In some cases a controller may deliver HCI-packets beyond
255 bytes.

This may for instance be the case if the controller delivers large ISO
SDUs as a single HCI-packet.

Signed-off-by: Johan Stridkvist <johan.stridkvist@nordicsemi.no>