[pvr] fix: wrong condition in PVRManager::IsIdle (Ticket #14630)

IsIdle() returns the wrong state (true) if delta between next timer and
now is equal to the backend idle time setting. In this condition XBMC
shuts down with a wrong wakeup time for the next timer.
Merge pull request #4208 from xhaggi/fix-pvr-idle-check

[pvr] fix: wrong condition in PVRManager::IsIdle (Ticket #14630)