arc: fix use of uninitialized arc_pmu

static arc_pmu in the arch/arc/kernel/perf_event.c is not initialized as
it's shadowed by a local variable of the same name in the
arc_pmu_device_probe.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Fixes: 03c94fcf

 "ARC: perf: make @arc_pmu static global"
CC: <stable@vger.kernel.org>	# 4.1
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>