Change uses of {to,from}Ascii to {to,from}Latin1

This operation should be a no-op anyway, since at this point in time,
the fromAscii and toAscii functions simply call their fromLatin1 and
toLatin1 counterparts.

Task-number: QTBUG-21872
Change-Id: I66018663366a886a1d34c8c9389536081a09f2b7
Reviewed-by: Paul Olav Tvete <paul.tvete@nokia.com>