[PATCH v2] irqchip/sifive-plic: Avoid signed shift in interrupt enable mask

[Pengpeng Hou]

Hi Radu, Thomas,

A brief follow-up on this v2. It corrects the Fixes tag as requested,
with the code change unchanged.

Please let me know whether it is queued or if any further changes are
needed.

Thanks,
Pengpeng