Ensure siop_acb alignment via malloc rather than let alignment depend

Ensure siop_acb alignment via malloc rather than let alignment depend
on the size of MI data structures that come before it in the softc.
Chip will not function properly if alignment is wrong.
Detected and fixed by: Chuck Cranor <chuck@ccrc.wustl.edu>
                and Michael L Hitch <osymh@gemini.oscs.montana.edu>