dma-mapping: check for failed IOVA allocation

Currently if we fail to allocate an IOVA from our VA pool we proceed to
map DMA_ERROR_CODE into the IOMMU, which is not what the client
requested or expected.  Fix this by checking for an error return value
while allocating an IOVA and bailing out as appropriate.

Change-Id: I13df7b9ab52536b757d8ae0ca74a5b9e1e1438b5
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>