ACPI / OSL: Clean up initrd table override code

This patch cleans up the initrd table override code by merging
redundant logics and re-ordering code blocks.

No functional changes.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>