mei: bus: Reset event_cb when disabling a device

After cancelling all reads from the disable hook, we need to reset the
event_cb pointer as well or else we won't be able to set a new one up
when re-enabling the device.
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/mei/bus.c

@@ -496,6 +496,8 @@ int mei_cl_disable_device(struct mei_cl_device *device)
 		}
 	}
 
+	device->event_cb = NULL;
+
 	mutex_unlock(&dev->device_lock);
 
 	if (!device->ops || !device->ops->disable)