iommu/arm-smmu: Use correct mask when printing SID during fault

As per the spec, FRSYNRA contains the SID in the lower 16 bits.
Currently we're masking off all but the lower 8 bits, so we could be
losing bits when we print SIDs.  Fix this by using a 16 bit mask.

Change-Id: I4a16c169a52d31b43b2f850a5558eecb64b93902
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>