Fix style in msi.c.

[iphydf]

• Don't use anonymous enums (typedef enum { ... } Name;).
• Don't use macros to generate structs (too magical, hard to grep).
• Assign output parameter once, and don't access it a lot in the
  function body.
• Don't pass type names as parameters to macros (this is C, we don't have
  templates, sorry).
• All function-like macros must be do-while(0).
• ++i instead of i++.
• No assignment-expressions.
• No void-casts.

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