dmaengine: rcar-dmac: clear pertinence number of channels

DMACHCLR clears each channels, but its channel number is based on
its SoC or IP. Current driver is using fixed 0x7fff (= for 15ch),
it is not good match for Gen3 or Gen2 Audio DMAC. This patch fixes it

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>