bcm63xx: ext_intc: fix warning

bcm63xx: ext_intc: fix warning

In file included from ./arch/mips/include/asm/io.h:34,
                 from ./arch/mips/include/asm/mmiowb.h:5,
                 from ./include/linux/spinlock.h:60,
                 from ./include/linux/irq.h:14,
                 from drivers/irqchip/irq-bcm6345-ext.c:10:
drivers/irqchip/irq-bcm6345-ext.c: In function 'bcm6345_ext_intc_of_init':
./arch/mips/include/asm/mach-bcm63xx/ioremap.h:48:9: warning: 'base' may be used uninitialized in this function [-Wmaybe-uninitialized]
  return is_bcm63xx_internal_registers((unsigned long)addr);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/irqchip/irq-bcm6345-ext.c:255:16: note: 'base' was declared here
  void __iomem *base;
                ^~~~

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
(cherry picked from commit 29c3bb5f413f0962971c66e6516b8409a78e0135)