Bluetooth: Controller: Collection of small fixes for BIS

Use PDU_BIS_LLID_FRAMED in empty PDUs for framed BIS

Allocate room for header when calculating max_pdu for framed BIS

Set group_sync_delay, stream_sync_delay and framed properly for BIS
broadcaster

Add missing call to isoal_tx_event_prepare() for BIS

Signed-off-by: Troels Nilsson <trnn@demant.com>
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>