greybus: control: Mode-switch is a 'core' operation
The operation layer allows only the 'core' operations on a connection,
which is in its 'disconnecting' state.
Mode switch is sent at the very end of interface-disable sequence, and
the control connection is already in its 'disconnecting' state at that
time. And so gb_operation_get_active() always fail with error -ENOTCONN.
The operation core already has support for such 'core' operations, just
that we were missing the needed flag while creating the mode switch
operation.