iommu: iommu-debug: Fix return code for iommu_debug_dma_unmap_write

iommu_debug_dma_unmap_write() returns 0 instead of an error code
when the command passed to the dma_unmap debugfs node is not
in the expected format.

Change-Id: Ie4f5dfa4423edc539fec9c38554e97c79fccc6c3
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>