ACPICA: Clarify ACPI_FREE_BUFFER usage.

Add a comment to clarify reason for using ACPI_FREE_BUFFER directly
instead of ACPI_FREE.

In addition to that, change one instance in which ACPI_FREE_BUFFER()
should be used instead of ACPI_FREE().

[rjw: Subject and changelog]
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>