[MIPS] BCM1480: Fix computation of interrupt mask address register.

  CC      arch/mips/sibyte/bcm1480/irq.o
arch/mips/sibyte/bcm1480/irq.c: In function 'bcm1480_mask_irq':
arch/mips/sibyte/bcm1480/irq.c:112: warning: cast to pointer from integer of different size
arch/mips/sibyte/bcm1480/irq.c:114: warning: cast to pointer from integer of different size
arch/mips/sibyte/bcm1480/irq.c: In function 'bcm1480_unmask_irq':
arch/mips/sibyte/bcm1480/irq.c:130: warning: cast to pointer from integer of different size
arch/mips/sibyte/bcm1480/irq.c:132: warning: cast to pointer from integer of different size

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