dmaengine: rcar-dmac: Reject zero-length slave DMA requests

While the .device_prep_slave_sg() callback rejects empty scatterlists,
it still accepts single-entry scatterlists with a zero-length segment.
These may happen if a driver calls dmaengine_prep_slave_single() with a
zero len parameter.  The corresponding DMA request will never complete,
leading to messages like:

    rcar-dmac e7300000.dma-controller: Channel Address Error happen

and DMA timeouts.

Although requesting a zero-length DMA request is a driver bug, rejecting
it early eases debugging.  Note that the .device_prep_dma_memcpy()
callback already rejects requests to copy zero bytes.

Reported-by: Eugeniu Rosca <[email protected]>
Analyzed-by: Yoshihiro Shimoda <[email protected]>
Signed-off-by: Geert Uytterhoeven <[email protected]>
Signed-off-by: Vinod Koul <[email protected]>

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