Bluetooth: host: ISO: Missing terminate reason in ISO disconnected callback

[cvinayak]

Is your enhancement proposal related to a problem? Please describe.
An application would like to cancel a prior invocation of bt_iso_big_sync using bt_iso_big_terminate leading to disconnected callback if the function succeeds to cancel.. Without a reason in the disconnected callback, application cannot differentiate a race between local cancellation of sync-in-progress, Remote disconnect by terminate procedure, or Sync loss.

See:
https:/zephyrproject-rtos/zephyr/blob/0445b53cbcaedea446faf3894d4bc20f58660b6d/samples/bluetooth/iso_receive/src/main.c#L343

Describe the solution you'd like
Provide disconnect reason

Describe alternatives you've considered
NA

Additional context
None.