dmaengine: sun6i: Fix impossible settings of burst and bus width

In the commit 1f9cd915b64bb95f ("dmaengine: sun6i: Fix memcpy operation"),
the signed values returned by convert_burst() and convert_buswidth()
were stored in an unsigned value.
Then, these values were considered as errors when non null.

As a result, DMA transfers were rejected when the burst or buswidth
had values different from 1, as 8 for the burst or 4 for the bus width.

Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>