iommu/mediatek: Check for NULL instead of IS_ERR()

of_platform_device_create() returns NULL on error, it never returns
error pointers.

Fixes: 0df4fabe ('iommu/mediatek: Add mt8173 IOMMU driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>