iommu: io-pgtable-arm: Only suppress map failures during map tests

We're currently suppressing all map failures during the entirety of the
selftests.  We really only want to suppress those failures during
individual negative map test cases to avoid logspam, but we *do* want to
see other map failures during the selftests.  Fix this by only
suppressing map failures during negative map test cases.

Change-Id: If51a95dd4d8c5b756cfa4597a5bdd7c75afe2637
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>