KVM: PPC: Book3S HV: Fix the set_one_reg for MMCR3

The kvmppc_set_one_reg_hv() wrongly get() the value
instead of set() for MMCR3. Fix the same.

Fixes: 5752fe0b ("KVM: PPC: Book3S HV: Save/restore new PMU registers")
Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/171759276847.1480.16387950124201117847.stgit@linux.ibm.com