dmaengine: sun6i: Enable additional burst lengths/widths on H3

The H3 supports bursts lengths of 1, 4, 8 and 16 transfers, each with
a width of 1, 2, 4 or 8 bytes.

The register value for the the width is log2-encoded, change the
conversion function to provide the correct value for width == 8.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>