ACPI: bus: Remove unneeded assignment

When acpi_kobj is NULL already, assigning NULL to it is redundant,
so don't do that.

Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>