irqchip/loongson-liointc: Fix misuse of gc->mask_cache

In gc->mask_cache bits, 1 means enabled and 0 means disabled, but in the
loongson-liointc driver mask_cache is misused by reverting its meaning.
This patch fix the bug and update the comments as well.

Fixes: dbb15226 ("irqchip: Add driver for Loongson I/O Local Interrupt Controller")
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/1596099090-23516-4-git-send-email-chenhc@lemote.com