irqchip/aspeed-i2c-ic: Fix return value check in aspeed_i2c_ic_of_init()

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

Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>