ACPI / osi: Cleanup _OSI("Linux") related code before introducing new support

This patch cleans up OSI code in osl.c to make osi_linux work for OSI
strings other than "Linux", so it can be re-used for other purposes.

Tested-by: Lukas Wunner <lukas@wunner.de>
Tested-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>