dmaengine: sprd: fix an NULL vs IS_ERR() bug

We recently cleaned this code up but we need to update the error
handling as well.  The devm_ioremap_resource() returns error pointers on
error, never NULL.

Fixes: e7f063ae ("dmaengine: sprd: Use devm_ioremap_resource() to map memory")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>