Fix conflict in sbdata0/sbautoincrement definition. (#507)

* Fix conflict in sbdata0/sbautoincrement definition.

Autoincrement only happens if an access actually takes place.

On the list, see subject:"sbaccess ambiguity".

* Only increment on successful reads.

This mirrors the write case, and allows debuggers to batch a large
number of reads and only check the error bits once, at the end.