media: rkvdec: Fix a NULL vs IS_ERR() bug in probe()

media: rkvdec: Fix a NULL vs IS_ERR() bug in probe()

The iommu_paging_domain_alloc() function doesn't return NULL on error it
returns error pointers.  Update the check and then set ->empty_domain to
NULL because the rest of the driver assumes it can be NULL.

Fixes: ff8c5622f9f7 ("media: rkvdec: Restore iommu addresses on errors")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Tested-by: Detlev Casanova <detlev.casanova@collabora.com>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>