crypto: qat - fix unmap invalid dma address

'dma_mapping_error' return a negative value if 'dma_addr' is equal to
'DMA_MAPPING_ERROR' not zero, so fix initialization of 'dma_addr'.

Signed-off-by: Hui Tang <tanghui20@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>