MIPS: lantiq: timer irq can be different to 7

The SVIP SoC has its timer IRQ on a different IRQ than 7. Fix up the irq
code to be able to handle this.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4229/