irqchip/loongson-eiointc: Skip handling if there is no pending interrupt

eiointc_irq_dispatch() iterates over the pending bit registers of the
interrupt controller and evaluates the result even if there is no interrupt
pending in a particular 64bit chunk.

Skip handling and especially the pointless write back for clearing the
non-pending bits if a chunk is empty.

[ tglx: Massaged changelog ]

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://lore.kernel.org/r/20240130082722.2912576-3-maobibo@loongson.cn