s390/dma: fix mapping_error detection

The map_page implementation of s390 returns DMA_ERROR_CODE in an error
situation. Correctly test if a mapping was erroneous (DMA_ERROR_CODE is
defined as ~0).

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>