sched/cpufreq_sched: modify pcpu_capacity handling

Use the cpu argument of cpufreq_sched_set_cap() to handle per_cpu writes,
as the thing can be called remotely (e.g., from load balacing code).

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Juri Lelli <juri.lelli@arm.com>