iommu/mediatek: fix device leak on of_xlate()

iommu/mediatek: fix device leak on of_xlate()

commit b3f1ee18280363ef17f82b564fc379ceba9ec86f upstream.

Make sure to drop the reference taken to the iommu platform device when
looking up its driver data during of_xlate().

Fixes: 0df4fabe208d ("iommu/mediatek: Add mt8173 IOMMU driver")
Cc: stable@vger.kernel.org	# 4.6
Acked-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Yong Wu <yong.wu@mediatek.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>