staging: comedi: adv_pci1710: tidy up digital input and output subdevice init

For aesthetics, add some whitespace to the digital input and output subdevice
init and rename the support functions. Also remove the unnecessary comment.

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>