device_link_add() returns NULL pointer not PTR_ERR() when it fails,
so replace the IS_ERR() check with NULL pointer check.

Fixes: 72f5801a ("dmaengine: fsl-edma: integrate v3 support")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20231129090000.841440-1-yangyingliang@huaweicloud.comSigned-off-by: Vinod Koul <vkoul@kernel.org>