powerpc/powernv: Unfreeze VF PE on releasing it

When releasing PE for SRIOV VF, the PE is forced to be frozen
wrongly. When the same PE is picked for another VF, it won't
work anyhow. The patch fixes the issue by unfreezing, not
freezing the VF PE when releasing it.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>