fpga: dfl: afu: Pass the correct device to dma_mapping_error()

dma_mapping_error() was being called on a different device struct than
what was passed to map/unmap.  Besides rendering the error checking
ineffective, it caused a debug splat with CONFIG_DMA_API_DEBUG.

Signed-off-by: Scott Wood <swood@redhat.com>
Acked-by: Wu Hao <hao.wu@intel.com>
Acked-by: Moritz Fischer <mdf@kernel.org>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>