PM: domains: Skip another warning in irq_safe_dev_in_sleep_domain()

In irq_safe_dev_in_sleep_domain() we correctly skip the dev_warn_once() if
the corresponding genpd for the device, has the GENPD_FLAG_ALWAYS_ON flag
being set. For the same reason (the genpd is always-on in runtime), let's
also skip the warning if the GENPD_FLAG_RPM_ALWAYS_ON flag is set for the
genpd.

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