irqchip/atmel-aic5: Use explicit variable name for the base chip

To avoid errors, use an explicit variable name when accessing the 'base'
generic chip.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Nicholas Ferre <nicolas.ferre@atmel.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: <sasha.levin@oracle.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Cc: <alexandre.belloni@free-electrons.com>
Cc: <Wenyou.Yang@atmel.com>
Cc: <jason@lakedaemon.net>
Cc: <marc.zyngier@arm.com>
Link: http://lkml.kernel.org/r/1442843173-2390-2-git-send-email-ludovic.desroches@atmel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>