dma: Document expected behavior of start/stop

The DMA API expects drivers in effect to maintain their own internal
state of channels. A channel that is already started may have dma_start
called on it again without effect. A channel that is already stopped may
have dma_stop called on it without effect.

In essence, start and stop are expected to act like events a DMA channels
finite state machine reacts to and if the state is already at the desired
one then nothing is to be done and no error has occurred.

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