bluetooth: controller: use correct CONFIG for central CIS create proc. data

Use CONFIG_BT_CTLR_CENTRAL_ISO instead of CONFIG_BT_CENTRAL in
declaring central CIS CREATE procedure context data. In some configs the
size of this data structure is bound by central CIS CREATE, so doing it
correctly will save memory in the procedure context allocation.

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