greybus: operation: add incoming-operation flag

Add flag field to struct gb_operation, and a first flag
GB_OPERATION_FLAG_INCOMING to identify incoming operations.

Pass an initial set of flags when allocating new operations, and use
these to identify incoming operations rather than overloading the
meaning of GB_OPERATION_TYPE_INVALID. This also allows us to set the
type for all operations during allocation.

Also add convenience helper to identify incoming operations.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>