MIPS: lantiq: Remove unnecessary include of <linux/of_irq.h>

arch/mips/pci/fixup-lantiq.c requires nothing from <linux/of_irq.h>, so
remove the include of it.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>