dmaengine: rcar-dmac: Add helpers for clearing DMA channel status

Extract the code to clear the status of one or all channels into their
own helpers, to prepare for the different handling of the R-Car V3U SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/20210128084455.2237256-4-geert+renesas@glider.be


Signed-off-by: Vinod Koul <vkoul@kernel.org>