dmaengine: mediatek: mtk-hsdma: use NULL instead of using plain integer as pointer

This fixes the following sparse warnings:
drivers/dma/mediatek/mtk-hsdma.c:604:26: warning: Using plain integer
as NULL pointer

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Link: https://lore.kernel.org/r/1649750340-30777-1-git-send-email-baihaowen@meizu.com


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