parisc: fix IOMMU's device boundary overflow bug on 32bits arch

On 32bits boxes, boundary_size becomes zero due to a overflow and we
hit BUG_ON in iommu_is_span_boundary.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Kyle McMartin <kyle@parisc-linux.org>
Cc: Matthew Wilcox <matthew@wil.cx>
Acked-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>