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>