sparc/iommu: merge iommu_release_one and sbus_iommu_unmap_page

There is only one caller of iommu_release_one left, so merge it into
that one to clean things up a bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>