The CPU needs time to change state (both direction). This patch adds a
timeout logic for that change to happen. If the change does not happen
within the timeout, then the driver returns a warning on the dmesg.

At this moment in time I'm not sure if it is worthy to actually return
an error. For now, to avoid API changes, we just print a warning.
Signed-off-by: Federico Vaga <federico.vaga@cern.ch>