ACPI: OSL: Rearrange workqueue selection in acpi_os_execute()

Replace the 3-branch if () statement used for selecting the target
workqueue in acpi_os_execute() with a switch () one that is more
suitable for this purpose and carry out the work item initialization
before it to avoid code duplication.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>