sync performs a strict superset of the functions of eieio, so using both
is redundant. While here, expand bus barriers to all bus_space operations,
since many drivers do not correctly use bus_space_barrier().
In principle, we can also replace sync just with eieio, for a significant
performance increase, but it remains to be seen whether any poorly-written
drivers currently depend on the side effects of sync to properly function.
MFC after: 1 week
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