irqchip: mtk-sysirq: Use IS_ERR() instead of NULL pointer check

Beniamino noticed a bug that an invalid DT file for the mediatek interrupt
polarity extension will cause kernel oops.

The reason is that the interrupt polarity support for mediatek chips
merely checks for NULL pointer instead of a casted error return
value in mtk_sysirq_of_init() so any other casted error value passes
the NULL pointer check and causes a kernel panic when dereferenced.

Use IS_ERR() and return the error value via PTR_ERR().

[ jac: took V2 over V3 for diff formatting, hand-added V3 changes,
tweaked subject line. ]

Reported-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Link: https://lkml.kernel.org/r/1418205302-22531-1-git-send-email-yingjoe.chen@mediatek.com


Signed-off-by: Jason Cooper <jason@lakedaemon.net>