staging: comedi: gsc_hpdi: tidy up hpdi_cmd()

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
interrupt support code.

Absorb the di_cmd() helper and tidy up the function a bit.

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>