irqchip/loongarch-cpu: Fix return value of lpic_gsi_to_irq()

lpic_gsi_to_irq() should return a valid Linux interrupt number if
acpi_register_gsi() succeeds, and return 0 otherwise. But lpic_gsi_to_irq()
converts a negative return value of acpi_register_gsi() to a positive value
silently.

Convert the return value explicitly.

Fixes: e8bba72b ("irqchip / ACPI: Introduce ACPI_IRQ_MODEL_LPIC for LoongArch")
Reported-by: Miao Wang <shankerwangmiao@gmail.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20240723064508.35560-1-chenhuacai@loongson.cn