dmaengine: ioat: set error code on failures

In function ioat_dma_self_test(), when the calls to dma_mapping_error()
fails, the value of return variable err is 0 (indicates no error). As a
result, the return value may be inconsistent with the execution status.
This patch fixes the bug by assigning -ENOMEM to err on the error path.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=188591


Signed-off-by: Pan Bian <bianpan2016@163.com>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>