Bluetooth: controller: add missing NTF alloc in central CIS Create

If a CIS create is requested prior to having a complete feature exchange
the central will initiate a feat exch before enqueing the CIS create.
IF then the feature exchange results in peripheral NOT supporting
CIS create the central needs to allocate an RX node for the NTF

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