ACPI / extlog: replace open-coded _DSM code with helper functions

Use helper functions to simplify _DSM related code in acpi_extlog driver.
Also mark initialization data and functions with __init and __initdata
to reduce memory footprint.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Tested-by: Chen, Gong <gong.chen@linux.intel.com>
Reviewed-by: Chen, Gong <gong.chen@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>