dmaengine: dw-axi-dmac: Fix RMW on channel suspend register

When the DMA is configured for more than 8 channels the bits controlling
suspend moves to another register. However when adding support for this
the new register would be completely overwritten in one case and
overwritten with values from the old register in another case.

Found by comparing the parallel implementation of more than 8 channel
support for the StarFive JH7100 SoC by Samin.

Fixes: 82435166

 ("dmaengine: dw-axi-dmac: support DMAX_NUM_CHANNELS > 8")
Co-developed-by: Samin Guo <samin.guo@starfivetech.com>
Signed-off-by: Samin Guo <samin.guo@starfivetech.com>
Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Link: https://lore.kernel.org/r/20220627090939.1775717-1-emil.renner.berthing@canonical.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>