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.

Fix that.

Tested on EVT 1.5 with gpbridge-test module.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Karthik Ravi Shankar <karthikrs@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>