PM / Domains: Respect errors from genpd's ->power_off() callback

The current code in genpd_sync_power_off(), doesn't care about potential
errors being returned from genpd's ->power_off() callback.

Obviously this behaviour could lead to problems, such as incorrectly
setting the genpd's status to GPD_STATE_POWER_OFF, but also to incorrectly
decrease the subdomain count for the masters, which potentially allows them
to be powered off in the next recursive call to genpd_sync_power_off().

Let's fix this behaviour by bailing out when the ->power_off() callback
returns an error code.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>