The reurn value of of_find_device_by_node() is not checked, thus null
pointer dereference will be triggered if of_find_device_by_node()
failed.

Fixes: bbe89c8e ("at_hdmac: move to generic DMA binding")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20200817115728.1706719-2-yukuai3@huawei.comSigned-off-by: Vinod Koul <vkoul@kernel.org>