staging: comedi: ni_tiocmd: remove BUG() which can never occur

All the counter_dev->variant options are handled by the switch.
Remove the BUG() 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>