irqchip/realtek-rtl: Service all pending interrupts

Instead of only servicing the lowest pending interrupt line, make sure
all pending SoC interrupts are serviced before exiting the chained
handler. This adds a small overhead if only one interrupt is pending,
but should prevent rapid re-triggering of the handler.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/5082ad3cb8b4eedf55075561b93eff6570299fe1.1641739718.git.sander@svanheule.net