hw/intc/loongarch_extioi: Fix typo issue about register EXTIOI_COREISR_END

hw/intc/loongarch_extioi: Fix typo issue about register EXTIOI_COREISR_END

Interrupt controller extioi supports 256 vectors, register EXTIOI_COREISR
records pending interrupt status with bitmap method. Size of EXTIOI_COREISR
is 256 / 8 = 0x20 bytes, EXTIOI_COREISR_END should be EXTIOI_COREISR_START
+ 0x20 rather than 0xB20.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20250605092848.1550985-1-maobibo@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>