[PARISC] Fix gcc 4.1 warnings in sba_iommu.c

Clean up gcc 4.1 warnings noted by Joel Soete.
Kyle McMartin gets kudos for pointing out the issues.
Matthew Wilcox noticed sba_iommu was using open coded versions
of (read|write)X.

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Matthew Wilcox <willy@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>