staging: comedi: gsc_hpdi: tidy up dio_config_insn()

For aesthetics, rename this function so it has namespace associated
with the driver and move it so that it is not in the middle of the
async command support code.

Absorb the dio_config_block_size() helper function and remove the
forward declaration.

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>