perf/powerpc: Fix build error

Fix compile errors like those below:

CC arch/powerpc/perf/power7-pmu.o
/home/git/linux/arch/powerpc/perf/power7-pmu.c:397:2: error: initialization from incompatible pointer type [-Werror]

Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Sukadev Bhattiprolu <[e-mail]>
Acked-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/powerpc/include/asm/perf_event_server.h

···
  * POWER CPU specification.
  */
 #define	EVENT_VAR(_id, _suffix)		event_attr_##_id##_suffix
-#define	EVENT_PTR(_id, _suffix)		&EVENT_VAR(_id, _suffix)
+#define	EVENT_PTR(_id, _suffix)		&EVENT_VAR(_id, _suffix).attr.attr
 
 #define	EVENT_ATTR(_name, _id, _suffix)					\
 	PMU_EVENT_ATTR(_name, EVENT_VAR(_id, _suffix), PME_PM_##_id,	\