microblaze: dma-mapping: support debug_dma_mapping_error

Add support for debug_dma_mapping_error() call to avoid warning from
debug_dma_unmap() interface when it checks for mapping error checked
status. Without this patch, device driver failed to check map error
warning is generated.

Signed-off-by: Shuah Khan <shuah.khan@hp.com>
Acked-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Joerg Roedel <joro@8bytes.org>