sparc64: Fix Niagara2 perf event handling.

sparc64: Fix Niagara2 perf event handling.

For chips like Niagara2 that have true overflow indications
in the %pcr (which we don't actually need and don't use)
the interrupt signal persists until the overflow bits are
cleared by an explicit %pcr write.

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