staging: comedi: ni_tio: tidy up ni_gpct_device_{construct,destroy)()

For aesthetics, move these exported functions to the end of the file.

Tidy up the functions a bit and remove the BUG_ON when (num_counters == 0).

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>