[le audio] Fix broadcast codec config if 48_2 is being used

If the source only support 48_2, we mistakenly falled back to 16_2
config. We should fix by checking if 48k config exist to use 48_2 config
instead.

Tag: #bug
Bug: 329134305
Flag: Exempt, small change to add additional config check, verified by QA locally
Test: manual testing with different quality settings
Test: atest bluetooth_le_audio_codec_manager_test

Change-Id: I40f2f7ee6c96a6a1babb7d87601db8cdc4e62b3a