PM / Runtime: Convert pm_runtime_set_suspended() to return an int

Because pm_runtime_set_suspended() invokes __pm_runtime_set_status(), which
can fail, pm_runtime_set_suspended() can also fail.

Instead of hiding a potential error, let's propagate it by converting
pm_runtime_set_suspended() from a void to return an int. In this way users
are able to check the error code and act accordingly.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>