s390/dma: support debug_dma_mapping_error

Without this patch drivers will get blamed (CONFIG_DMA_API_DEBUG=y)
for not calling dma_mapping_error (even if they do).

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Acked-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>