sparc64: Fix maybe_change_configuration() PCR setting.

Need to mask out the existing event bits before OR'ing in
the new ones.

Noticed by Peter Zijlstra.
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc/kernel/perf_event.c

@@ -655,6 +655,7 @@ static u64 maybe_change_configuration(struct cpu_hw_events *cpuc, u64 pcr)
 		cpuc->current_idx[i] = idx;
 
 		enc = perf_event_get_enc(cpuc->events[i]);
+		pcr &= ~mask_for_index(idx);
 		pcr |= event_encoding(enc, idx);
 	}
 out: