Bluetooth: controller: Introduce CIS LLL flush states

To keep track of requested/pending CIS LLL flushing, change 'flushed'
binary state to 'flush' with states NONE, PENDING and COMPLETE.

This enables CIS teardown to know that a CIS already has a pending LLL
flush, and ULL does not need to initiate it.

Signed-off-by: Morten Priess <mtpr@oticon.com>