dmaengine: ioat: set error code on failures

In function ioat_xor_val_self_test(), when the calls to
dma_mapping_error() fail, 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=188601


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>