irqchip/qcom: Fix error handling

'devm_ioremap()' returns NULL on error, not an error pointer.

Fixes: f20cc9b0 ("irqchip/qcom: Add IRQ combiner driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: marc.zyngier@arm.com
Cc: kernel-janitors@vger.kernel.org
Cc: jason@lakedaemon.net
Link: http://lkml.kernel.org/r/20170218083434.2289-1-christophe.jaillet@wanadoo.fr


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