KVM: PPC: Book3s HV: Fix tlbie compile error

Some compilers complain about uninitialized variables in the compute_tlbie_rb
function. When you follow the code path you'll realize that we'll never get
to that point, but the compiler isn't all that smart.

So just default to 4k page sizes for everything, making the compiler happy
and the code slightly easier to read.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Paul Mackerras <paulus@samba.org>