MIPS: IP27: Fix build errors with CONFIG_PCI disabled.

  LD      init/built-in.o
arch/mips/built-in.o: In function `startup_bridge_irq':
ip27-irq.c:(.text+0x434): undefined reference to `irq_to_slot'
ip27-irq.c:(.text+0x43c): undefined reference to `irq_to_slot'
ip27-irq.c:(.text+0x460): undefined reference to `irq_to_bridge'
ip27-irq.c:(.text+0x464): undefined reference to `irq_to_bridge'
arch/mips/built-in.o: In function `shutdown_bridge_irq':
ip27-irq.c:(.text+0x564): undefined reference to `irq_to_bridge'
ip27-irq.c:(.text+0x56c): undefined reference to `irq_to_bridge'
ip27-irq.c:(.text+0x5a0): undefined reference to `irq_to_slot'
ip27-irq.c:(.text+0x5a4): undefined reference to `irq_to_slot'

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