irqchip/riscv-intc: Prevent memory leak when riscv_intc_init_common() fails

When riscv_intc_init_common() fails, the firmware node allocated is not
freed. Add the missing free().

Fixes: 7023b9d83f03 ("irqchip/riscv-intc: Add ACPI support")
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Anup Patel <anup@brainfault.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240527081113.616189-1-sunilvl@ventanamicro.com