dmaengine: owl: fix register access functions

When building with 'make W=1', clang notices that the computed register
values are never actually written back but instead the wrong variable
is set:

drivers/dma/owl-dma.c:244:6: error: variable 'regval' set but not used [-Werror,-Wunused-but-set-variable]
  244 |         u32 regval;
      |             ^
drivers/dma/owl-dma.c:268:6: error: variable 'regval' set but not used [-Werror,-Wunused-but-set-variable]
  268 |         u32 regval;
      |             ^

Change these to what was most likely intended.

Fixes: 47e20577

 ("dmaengine: Add Actions Semi Owl family S900 DMA driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20240322132116.906475-1-arnd@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>