staging: comedi: ni_660x: remove dma_selection_counter()

The counter->chip_index will always be < counters_per_chip due to the
initialization of the subdevices during the attach of the board.

The dma_selection_counter() helper just does a BUG_ON() check before
returning the original value. Just use the original value directly
in the caller and remove the helper function.

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>