ACPI/IORT: Fix the error return code in iort_add_smmu_platform_device()

The function iort_add_smmu_platform_device() accidentally returns 0
(ie PTR_ERR(pdev) where pdev == NULL) if platform_device_alloc() fails;
fix the bug by returning a proper error value.

Fixes: 846f0e9e ("ACPI/IORT: Add support for ARM SMMU platform devices creation")
Acked-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[lorenzo.pieralisi@arm.com: improved commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>

Signed-off-by: Will Deacon <will.deacon@arm.com>