ACPI: AC: Remove redundant checks

Remove unnecessary checks against NULL for pointers that can't be NULL
when the checks are done.

Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>