dma: Test repeatedly calling start/stop

Ensures that the documented behavior of the API is met by implementations
through testing. By calling stop on a stopped channel the expectation is no
error occurs and is checked.

Calling start after a channel has been started is difficult to test for
as there is transfer timing involved. A once shot transfer may have
completed and the channel transition to an inactive state by itself by
the time the second start call is performed. This isn't supported by at
least gpdma today and isn't documented behaviorally so should not be
tested.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>