[POWERPC] Fix bug with early ioremap and 64k pages

The code for bolting hash entries for ioremap done before proper
mm initialization has a grown a bug when using 64K pages on a
machine where non-cacheable mappings are demoted to 4K HW pages.
The wrong page size index is being passed to the hash table mapping
functions causing a crash at boot on some pSeries machines using
bare metal linux.  This fixes it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>