xtensa: simplify xtensa_pmu_irq_handler

Use for_each_set_bit() instead of open-coding it to simplify the code.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Message-Id: <20200708062023.7986-1-vulab@iscas.ac.cn>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>