Bluetooth: controller: fixing CIS establish flow for refactored LLCP

Latest modification to LLCP removed 'monitoring' of cis->established
flag in LLCP, relying instead on signal through ULL.

For this to work properly the signal must also be generated, so this is
introduced. Note - checking of cis->established flag could be removed
if caller ensures only calling once (on establish condition)

Signed-off-by: Erik Brockhoff <erbr@oticon.com>