staging: comedi: gsc_hpdi: rename abort_dma()

For aesthetics, rename this function so it has namespace associated
with the driver.

Also, remove the unnecessary 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>