Bluetooth: Controller: Add LL_CIS_HANDLE_LAST, IDX_FROM_HANDLE define

Rename LAST_VALID_CIS_HANDLE to LL_CIS_HANDLE_LAST, to be
consistent naming with LL_CIS_HANDLE_BASE.
Add IDX_FROM_HANDLE defines.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>