iommu/amd: Remove unnecessary assignment

From: Adrian Huang <ahuang12@lenovo.com>

The values of local variables are assigned after local variables
are declared, so no need to assign the initial value during the
variable declaration.

And, no need to assign NULL for the local variable 'ivrs_base'
after invoking acpi_put_table().

Signed-off-by: Adrian Huang <ahuang12@lenovo.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20201210021330.2022-1-adrianhuang0701@gmail.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>