powerpc/64s: Fix mce accounting for powernv

On 64-bit Book3s, when we're in HV mode, we have already counted the
machine check exception in machine_check_early().

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Use IS_ENABLED() rather than an #ifdef]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>