iommu: msm_iommu: Return proper value from unmap_range

unmap_range should return an error code, not just zero.
Technially the return code should be the number of bytes
unmapped but certain kernel clients spew errors because
of incorrect error handling. Until that can be filed,
return negative error codes to indicate an error.

Change-Id: Iae0b561c99edc216d3d484107be5938f3775e11d
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>