Fix for regression issue in USB keyboard driver after r304735.

A series of zero delay callouts can happen causing high CPU usage of the
timer subsystem when trying to repeat keys, because the time of the
absolute timeout is not moving forward. The condition clears when all
keys are released.

Reported by: Johannes Lundberg <johalun0@gmail.com>
Discussed with: bde@
PR: 226968
MFC after: 1 week
Sponsored by: Mellanox Technologies