PM: runtime: Make clear what we do when conditions are wrong in rpm_suspend()

rpm_suspend() simple bails out when conditions are wrong. But this is not
immediately obvious from the code.  Make it clear what we do when conditions
are wrong in rpm_suspend().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>