irqchip/irq-qcom-mpm: fix return value check in qcom_mpm_init()

If devm_platform_ioremap_resource() fails, it never returns
NULL, replace NULL check with IS_ERR().

Fixes: a6199bb5

 ("irqchip: Add Qualcomm MPM controller driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220316025100.1758413-1-yangyingliang@huawei.com