PM-runtime: Fix 'jiffies' in comments after switch to hrtimers

PM-runtime now uses the hrtimers infrastructure for autosuspend, however
comments still reference 'jiffies'.

Fixes: 8234f673 (PM-runtime: Switch autosuspend over to using hrtimers)
Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>