PPC: booke206: move avail check to tlbwe

We can have TLBs that only support a single page size. This is defined
by the absence of the AVAIL flag in TLBnCFG. If this is the case, we
currently write invalid size info into the TLB, but override it on
internal fault.

Let's move the check over to tlbwe, so we don't have the AVAIL check in
the hotter fault path.

Signed-off-by: Alexander Graf <agraf@suse.de>
