Fix a paste-o.

Submitted by:		Michal Meloun <meloun@miracle.cz>
Change the order of operations for the initial cache setup.  Turning off
the cache before clean/invalidate ensured that no new lines can come into
the cache or migrate between levels during the operation, but may not be
safe on some chips.  Instead, if the cache was enabled on entry, do the
wbinv while it's still enabled, and then disable it and do a separate
invalidate pass.  After the intitial writeback we know there are no
dirty lines left and no new dirty lines can be created as long as we
carefully avoid touching memory before turning the cache off.  Add a
comment about that so no new code gets inserted between those points.