The ioremap() function doesn't return error pointers, it returns NULL
on error so update the error handling.  Also just return directly
instead of calling iounmap() on the NULL pointer.  Calling
iounmap(NULL) doesn't cause a problem on ARM but on other architectures
it can trigger a warning so it'a bad habbit.

Fixes: 918eb5c8 ("iommu/arm-smmu-v3: Add in-kernel support for NVIDIA Tegra241 (Grace) CMDQV")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Link: https://lore.kernel.org/r/5a6c1e9a-0724-41b1-86d4-36335d3768ea@stanley.mountainSigned-off-by: Will Deacon <will@kernel.org>