Bluetooth: ISO: Fix missing fallback to HCI for ISO data path

A recent change broke the fallback to using a simple
HCI data path configuration in case that the application
does not provide a data path.

This commit fixes that issue, while retaining the
intended update from the change that broke the fallback,
as well as making the code a bit easier to read
and more documented.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>