target-ppc: fix left shift overflow in hpte_page_shift

ps->pte_enc is a 32-bit value, which is shifted left and then compared
to a 64-bit value.  It needs a cast before the shift.

Reported by Coverity.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>