kvm/ppc/e500: g2h_tlb1_map: clear old bit before setting new bit

It's possible that we're using the same host TLB1 slot to map (a
presumably different portion of) the same guest TLB1 entry.  Clear
the bit in the map before setting it, so that if the esels are the same
the bit will remain set.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>