media: iommu/mediatek: Return ENODEV if the device is NULL

The platform device is created at:
of_platform_default_populate_init:  arch_initcall_sync
  ->of_platform_populate
        ->of_platform_device_create_pdata

When entering our probe, all the devices should be already created.
if it is null, means NODEV. Currently we don't get the fail case.
It's a minor fix, no need add fixes tags.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Acked-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>