PM / Domains: Correct comment in irq_safe_dev_in_no_sleep_domain()

The earlier comment stated that the dev_warn_once() was going to be printed
once per device. Let's fix that, as dev_warn_once() is printed only once,
no matter of the device.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Lina Iyer <lina.iyer@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>