PM / Domains: Don't return -EEXIST at attach when PM domain exists

As dev_pm_domain_attach() isn't the only way to assign PM domain pointers
to devices, clearly we must allow a device to have the pointer already
being assigned. For this reason, return 0 instead of -EEXIST.

Reported-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Tested-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/power/common.c

@@ -106,7 +106,7 @@ int dev_pm_domain_attach(struct device *dev, bool power_on)
 	int ret;
 
 	if (dev->pm_domain)
-		return -EEXIST;
+		return 0;
 
 	ret = acpi_dev_pm_attach(dev, power_on);
 	if (!ret)