Change remaining uses of {to,from}Ascii to {to,from}Latin1 [QPA plugins]

Change remaining uses of {to,from}Ascii to {to,from}Latin1 [QPA plugins]

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: I4aef078ea87ecd3699de107d1e9352d9fb6d6e39
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@nokia.com>