LeAudio/Broadcast: Fix channel map creation

There were to many channels enabled in the stream map for each BIS
instead of dividing the channels between all the BISes.

Bug: 335809981
Test: atest bluetooth_le_audio_codec_manager_test
Flag: EXEMPT; Regression fix covered with unit test
Change-Id: I74132a77993f97b5ea7ea5c200b749728048e275