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.