KVM: PPC: e500: Silence bogus GCC warning in tlb code

64-bit GCC 4.5.1 warns about an uninitialized variable which was guarded
by a flag. Initialize the variable to make it happy.

Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
[agraf: reword comment]
Signed-off-by: Alexander Graf <agraf@suse.de>