staging: comedi: ni_660x: remove BUG_ON() in ni_660x_request_mite_channel()

This BUG_ON() happens if a mite DMA channel is already requested when an
ansynchronous command is started for one of the counter subdevices.

The comedi core will only call the (*do_cmd) if the subdevice is not busy.
In this driver, the (*cancel) for the subdevice will always release any
requested mite DMA channel.

Remove the BUG_ON() which can never occur.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/ni_660x.c

@@ -599,7 +599,6 @@ static int ni_660x_request_mite_channel(struct comedi_device *dev,
 	struct mite_channel *mite_chan;
 
 	spin_lock_irqsave(&devpriv->mite_channel_lock, flags);
-	BUG_ON(counter->mite_chan);
 	mite_chan = mite_request_channel(devpriv->mite,
 					 mite_ring(devpriv, counter));
 	if (!mite_chan) {