irqchip/sifive-plic: Make use of __assign_bit()

Replace the open coded

if (foo)
        __set_bit(n, bar);
    else
        __clear_bit(n, bar);

with __assign_bit(). No functional change intended.

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/all/20240902130824.2878644-1-lihongbo22@huawei.com