iommu: Unexport iommu_fwspec_free()

The drivers doing their own fwspec parsing have no need to call
iommu_fwspec_free() since fwspecs were moved into dev_iommu, as
returning an error from .probe_device will tear down the whole lot
anyway. Move it into the private interface now that it only serves
for of_iommu to clean up in an error case.

I have no idea what mtk_v1 was doing in effectively guaranteeing
a NULL fwspec would be dereferenced if no "iommus" DT property was
found, so add a check for that to at least make the code look sane.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/36e245489361de2d13db22a510fa5c79e7126278.1740667667.git.robin.murphy@arm.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>