MIPS: Oprofile: Fix Loongson irq handler

    The interrupt enable bit for the performance counters is in the Control
    Register $24, not in the counter register.
    loongson2_perfcount_handler(), we need to use

    Reported-by: Xu Hengyang <hengyang@mail.ustc.edu.cn>
    Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
    Cc: linux-mips@linux-mips.org
    Patchwork: http://patchwork.linux-mips.org/patch/1198/
    Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
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