dmaengine: sun6i: Fix the access of the IRQ register

The IRQ register number is computed, but this number was not used
and the register was the one indexed by the channel index instead.
Then, only the first DMA channel was working.

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>