irqchip/loongson-liointc: Fix improper error handling in liointc_init()

For cores less than 4, eg, loongson2k1000 with 2 cores, the
of_property_match_string() may return with an error value,
which causes that liointc could not work. At least isr0 is
what should be checked like previous commit b2c4c396
("irqchip/loongson-liointc: irqchip add 2.0 version") did.

Fixes: 0858ed03 ("irqchip/loongson-liointc: Add ACPI init support")
Signed-off-by: Liu Peibao <liupeibao@loongson.cn>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221104110712.23300-1-liupeibao@loongson.cn