iommu/omap: Fix regression in probe for NULL pointer dereference

Commit 3f6634d9 ("iommu: Use right way to retrieve iommu_ops") started
triggering a NULL pointer dereference for some omap variants:

__iommu_probe_device from probe_iommu_group+0x2c/0x38
probe_iommu_group from bus_for_each_dev+0x74/0xbc
bus_for_each_dev from bus_iommu_probe+0x34/0x2e8
bus_iommu_probe from bus_set_iommu+0x80/0xc8
bus_set_iommu from omap_iommu_init+0x88/0xcc
omap_iommu_init from do_one_initcall+0x44/0x24

This is caused by omap iommu probe returning 0 instead of ERR_PTR(-ENODEV)
as noted by Jason Gunthorpe <jgg@ziepe.ca>.

Looks like the regression already happened with an earlier commit
6785eb91

 ("iommu/omap: Convert to probe/release_device() call-backs")
that changed the function return type and missed converting one place.

Cc: Drew Fustini <dfustini@baylibre.com>
Cc: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Suman Anna <s-anna@ti.com>
Suggested-by: Jason Gunthorpe <jgg@ziepe.ca>
Fixes: 6785eb91 ("iommu/omap: Convert to probe/release_device() call-backs")
Fixes: 3f6634d9

 ("iommu: Use right way to retrieve iommu_ops")
Signed-off-by: Tony Lindgren <tony@atomide.com>
Tested-by: Drew Fustini <dfustini@baylibre.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20220331062301.24269-1-tony@atomide.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>