iommu/msm: Fix error handling in msm_iommu_unmap()

Error handling in msm_iommu_unmap() is broken. On some error
conditions retval is set to a non-zero value which causes
the function to return 'len' at the end. This hides the
error from the user. Zero should be returned in those error
cases.

Cc: David Brown <davidb@codeaurora.org>
Cc: Stepan Moskovchenko <stepanm@codeaurora.org>
Cc: stable@vger.kernel.org # >= 3.1
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Acked-by: David Brown <davidb@codeaurora.org>