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

In the Linux kernel, the following vulnerability has been resolved:

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

Commit 3f6634d997db ("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
6785eb9105e3 ("iommu/omap: Convert to probe/release_device() call-backs")
that changed the function return type and missed converting one place.