iommu/sun50i: Fix return value check in sun50i_iommu_probe()

iommu/sun50i: Fix return value check in sun50i_iommu_probe()

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

Fixes: 4100b8c229b3 ("iommu: Add Allwinner H6 IOMMU driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20200519091857.134170-1-weiyongjun1@huawei.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>