PM: domains: Don't check PM_QOS_FLAG_NO_POWER_OFF in genpd

Back in the days when genpd supported intermediate power states of its
devices, it made sense to check the PM_QOS_FLAG_NO_POWER_OFF in
genpd_power_off(). This because the attached devices were all being put
into low power state together when the PM domain was also being powered
off.

At this point, the flag PM_QOS_FLAG_NO_POWER_OFF is better checked by
drivers from their ->runtime_suspend() callbacks, like in the
usb_port_runtime_suspend(), for example. Or perhaps an even better option
is to set the QoS resume latency constraint for the device to zero, which
informs the runtime PM core to prevent the device from being runtime
suspended.

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