MIPS: BCM63XX: Change irq code to prepare for per-cpu peculiarity.

No functionnal change is introduced by this patch.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2894/


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