MIPS: ralink: add missing comment in irq driver

Trivial patch that adds a comment that makes the code more readable.

Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5168/