xtensa: fix xtensa_pmu_setup prototype

Fix the following build error in configurations with
CONFIG_XTENSA_VARIANT_HAVE_PERF_EVENTS=y:

  arch/xtensa/kernel/perf_event.c:420:29: error: passing argument 3 of
  ‘cpuhp_setup_state’ from incompatible pointer type

Cc: stable@vger.kernel.org
Fixes: 25a77b55

 ("xtensa/perf: Convert the hotplug notifier to state machine callbacks")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>