mailbox: arm_mhuv2: Fix return value check in mhuv2_probe()

If devm_of_iomap() fails, it returns ERR_PTR() and never
return NULL, so replace NULL pointer check with IS_ERR()
to fix this problem.

Fixes: 5a6338cc ("mailbox: arm_mhuv2: Add driver")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>