iommu/dma: Cleanup dma-iommu.h

No need for a __KERNEL__ guard outside uapi and add a missing comment
describing the #else cpp statement.  Last but not least include
<linux/errno.h> instead of the asm version, which is frowned upon.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>