hw/intc: Fix LoongArch extioi coreisr accessing

1. When cpu read or write extioi COREISR reg, it should access
the reg belonged to itself, so the cpu index of 's->coreisr'
is current cpu number. Using MemTxAttrs' requester_id to get
the cpu index.
2. it need not to mask 0x1f when calculate the coreisr array index.

Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20221021015307.2570844-3-yangxiaojuan@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>