irqchip/loongson-htvec: Fix initial interrupt clearing

In htvec_reset() only the first group of initial interrupts is cleared.
This sometimes causes spurious interrupts, so let's clear all groups.

While at it, fix the nearby comment that to match the reality of what
the driver does.

Fixes: 818e915f ("irqchip: Add Loongson HyperTransport Vector support")
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/1599819978-13999-2-git-send-email-chenhc@lemote.com