Bluetooth: Controller: Fix uninitialized CIS LLL member variables

Fix uninitialized CIS LLL structure member variables,
uninitialised CIS acl_handle causes HCI command disallowed
for acl_handle's other than 0x0000.
Initialize other variables like sn, nesn etc. when create
CIS control procedure is performed.

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