iommu/arm-smmu: Limit 2-level strtab allocation for small SID sizes

If the StreamIDs in a system can all be resolved by a single level-2
stream table (i.e. SIDSIZE < SPLIT), then we currently get our maths
wrong and allocate the largest strtab we support, thanks to unsigned
overflow in our calculation.

This patch fixes the issue by checking the SIDSIZE explicitly when
calculating the size of our first-level stream table.

Reported-by: Matt Evans <matt.evans@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>