device_suspend — Save state and stop all devices in system.
int device_suspend (pm_message_t state);
Walk the dpm_active list, call ->suspend for each device, and move
it to dpm_off.
Check the return value for each. If it returns 0, then we move the
the device to the dpm_off list. If it returns -EAGAIN, we move it to
the dpm_off_irq list. If we get a different error, try and back out.
If we hit a failure with any of the devices, call device_resume
above to bring the suspended devices back to life.