PM: domains: Rename irq_safe_dev_in_no_sleep_domain() in genpd

The name "irq_safe_dev_in_no_sleep_domain", doesn't really match the
conditions that are being checked in the function, hence the code becomes a
bit confusing to read.

Let's clarify this by renaming it into "irq_safe_dev_in_sleep_domain" and
let's also take the opportunity to clarify a corresponding comment in the
code.

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