ACPI: bus: Remove the unneeded result variable

Return the value from driver_register() directly instead of storing it
in another redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>