powerpc/mm: Unindent htab_dt_scan_page_sizes()

We can unindent the bulk of htab_dt_scan_page_sizes() by returning early
if the property is not found. That is nice in and of itself, but also
has the advantage of making it clear that we always return success once
we have found the ibm,segment-page-sizes property.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>