staging: comedi: ni_mio_common: remove some unnecessary BUG_ON()

The mite dma channels are only requested by a subdevice with a (*do_cmd)
and they are released by a (*cancel). The comedi core will only call the
(*do_cmd) if the subdevice is not already running a command and all
commands are terminated by the core with a (*cancel).

Remove the BUG_ON() checks in the request functions which can never
happen.

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>