ACPI / memhotplug: Use defined marco METHOD_NAME__STA

We already have predefined marco for method name "_STA', so
using the marco instead of directly using the string.

Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>