irqchip/mxs: Fix error check of of_io_request_and_map()

The of_io_request_and_map() returns a valid pointer in iomem region or
ERR_PTR(), check for NULL always fails and may cause a NULL pointer
dereference on error path.

Fixes: 25e34b44 ("irqchip/mxs: Prepare driver for hardware with different offsets")
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Oleksij Rempel <linux@rempel-privat.de>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1457486500-10237-1-git-send-email-vz@mleia.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>