iommu/exynos: Fix warnings from DMA-debug

Add a simple checks for dma_map_single() return value to make DMA-debug
checker happly. Exynos IOMMU on Samsung Exynos SoCs always use device,
which has linear DMA mapping ops (dma address is equal to physical memory
address), so no failures are returned from dma_map_single().

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>