msm: iommu: Miscellaneous code cleanup

Remove some unneeded assignments and messages, restructure
a failure path in iova_to_phys, and make __flush_iotlb
return int in preparation for adding IOMMU clock control.

Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>
Signed-off-by: Daniel Walker <dwalker@codeaurora.org>