[PATCH] b44: race on device closing

Usual fix:
- b44_interrupt() does not schedule NAPI polling when the device is
  going down;
- b44_close() waits for any scheduled NAPI polling before it starts
  to release the private structures of the device.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>