irqchip/loongson-eiointc: Fix return value checking of eiointc_index

Return value of function eiointc_index is int, however it is converted
into uint32_t and then compared smaller than zero, this will cause logic
problem.

Fixes: dd281e1a ("irqchip: Add Loongson Extended I/O interrupt controller support")
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230811095805.2974722-2-maobibo@loongson.cn