irqchip/exiu: Fix return value check in exiu_init()

In case of error, the function of_iomap() returns NULL pointer not
ERR_PTR().

Replace the IS_ERR() test of the return value with NULL test and return
a proper error code.

Fixes: 706cffc1 ("irqchip/exiu: Add support for Socionext Synquacer EXIU controller")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Link: https://lkml.kernel.org/r/1510642648-123574-1-git-send-email-weiyongjun1@huawei.com