iommu/omap: ->unmap() should return order of unmapped page

Users of the IOMMU API (kvm specifically) assume that iommu_unmap()
returns the order of the unmapped page.

Fix omap_iommu_unmap() to do so and adopt omap-iovmm accordingly.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>