irqchip: mtk-sysirq: Make mtk_sysirq_of_init return error if ioremap fails

irqchip: mtk-sysirq: Make mtk_sysirq_of_init return error if ioremap fails

PTR_ERR(NULL) returns 0 so current code returns 0 if ioremap fails, fix it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Yingjoe Chen <yingjoe.chen@mediatek.com>
Cc: linux-mediatek@lists.infradead.org <linux-mediatek@lists.infradead.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Link: http://lkml.kernel.org/r/1432220254.29544.1.camel@ingics.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>