perf_counter: Fix atomic_set vs. atomic64_t type mismatch

Using atomic_set on an atomic64_t variable gives a compiler
warning on powerpc, and won't give the desired result at runtime.
This fixes an instance of this error in the perf_counter code.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <18995.20490.979429.244883@cargo.ozlabs.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>