irqchip/irq-rda-intc: Fix return value check in rda8810_intc_init()

In case of error, the function of_io_request_and_map() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().

Fixes: d852e62a ("irqchip: Add RDA8810PL interrupt driver")
Acked-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>