iommu/exynos: return 0 if iommu_attach_device() successes

iommu/exynos: return 0 if iommu_attach_device() successes

iommu_attach_device() against exynos-iommu positive integer on success
if the caller calls iommu_attach_device() with the same iommu_domain
multiple times without call to iommu_detach_device() to inform the
caller how many calls to iommu_detach_device() to really detach iommu.

However the convention of the return value of success of common API is
zero, this patch makes iommu_attach_device() call against exynos-iommu
always return zero if the given device is successfully attached to
the given iommu_domain even though it is already attached to the same
iommu_domain.

Signed-off-by: Cho KyongHo <pullip.cho@samsung.com>