irqchip/ompic: fix return value check in ompic_of_init()

In case of error, the function ioremap() returns NULL pointer not
ERR_PTR(). The IS_ERR() test in the return value check should be
replaced with NULL test.

Fixes: 9b54470a ("irqchip: add initial support for ompic")
Acked-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>