irqchip/irq-mvebu-sei: Fix a NULL vs IS_ERR() bug in probe function

The devm_ioremap_resource() function never returns NULL, it returns
error pointers.

Fixes: 61ce8d8d

 ("irqchip/irq-mvebu-sei: Add new driver for Marvell SEI")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Gregory Clement <gregory.clement@bootlin.com>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: kernel-janitors@vger.kernel.org
Link: https://lkml.kernel.org/r/20181013102246.GD16086@mwanda