MIPS: Lantiq: Fix external interrupt sources

The irq base offset needs to be ignored when matching irqs to external
interrupt pins. Taking the offset into account resulted in the EIU not
being brought up properly.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2616/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>