Bluetooth: Controller: Fix Broadcast ISO PDU receive connection handle

Fix broadcast ISO PDU receive connection handle. The look up
was using BIS index instead of the index of the selected
stream that is being received.

Example, when receiving BISes 2 and 4, only the connection
handle for BIS 2 was correct and connection handle lookup
was out-of-bounds for BIS 4; symptoms being that Host
dropped the received ISO PDU.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>